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Journal of Bacteriology, February 2007, p. 761-771, Vol. 189, No. 3
0021-9193/07/$08.00+0     doi:10.1128/JB.01327-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Novel Type IV Secretion System Involved in Propagation of Genomic Islands

Mario Juhas,^1* Derrick W. Crook,¹ Ioanna D. Dimopoulou,¹ Gerton Lunter,² Rosalind M. Harding,³ David J. P. Ferguson,⁴ and Derek W. Hood⁵

Clinical Microbiology and Infectious Diseases, Nuffield Department of Clinical Laboratory Sciences, University of Oxford, OX3 9DU Oxford, United Kingdom,¹ MRC Functional Genetics Unit, Department of Physiology, Anatomy and Genetics, University of Oxford, OX1 3PT Oxford, United Kingdom,² Departments of Zoology and Statistics, University of Oxford, OX1 3TG Oxford, United Kingdom,³ Department of Pathology, University of Oxford, OX3 9DU Oxford, United Kingdom,⁴ Molecular Infectious Diseases Group, The Weatherall Institute of Molecular Medicine, University of Oxford, OX3 9DS Oxford, United Kingdom⁵

Received 21 August 2006/ Accepted 10 November 2006

Top Abstract Introduction Materials and Methods Results Discussion References

 
Type IV secretion systems (T4SSs) mediate horizontal gene transfer, thus contributing to genome plasticity, evolution of infectious pathogens, and dissemination of antibiotic resistance and other virulence traits. A gene cluster of the Haemophilus influenzae genomic island ICEHin1056 has been identified as a T4SS involved in the propagation of genomic islands. This T4SS is novel and evolutionarily distant from the previously described systems. Mutation analysis showed that inactivation of key genes of this system resulted in a loss of phenotypic traits provided by a T4SS. Seven of 10 mutants with a mutation in this T4SS did not express the type IV secretion pilus. Correspondingly, disruption of the genes resulted in up to 100,000-fold reductions in conjugation frequencies compared to those of the parent strain. Moreover, the expression of this T4SS was found to be positively regulated by one of its components, the tfc24 gene. We concluded that this gene cluster represents a novel family of T4SSs involved in propagation of genomic islands.

Top Abstract Introduction Materials and Methods Results Discussion References

 
The horizontal gene pool contributes to the diversification and adaptation of microorganisms. A major part of the horizontal gene pool consists of genomic islands. Genomic islands are well-characterized segments of bacterial genomes, often inserted at tRNA genes, that contain homologues of genes encoding integrases and other genes associated with conjugative plasmids or phages (17).

Prior to the 1970s Haemophilus influenzae was universally susceptible to ampicillin. In 1972, the first ampicillin-resistant isolate was detected, and soon after this strains resistant to tetracycline, chloramphenicol, erythromycin, and multiple antibiotics were identified and spread rapidly around the globe. Work over the past few years has provided evidence that horizontal transfer of genes in bacteria, including transfer of antibiotic resistance, is facilitated by genomic islands.

Genomic islands of many bacterial plant and animal pathogens encode type IV secretion systems (T4SSs) which are preferentially used for delivery of bacterial effector proteins across the bacterial membrane and the plasmatic membrane into the eukaryotic host cells (4, 18, 24, 28). T4SSs also mediate horizontal gene transfer, thus contributing to genome plasticity, the evolution of infectious pathogens, and dissemination of antibiotic resistance and other virulence traits (9, 22). The structures of the genetic determinants of T4SSs vary and consist of multiple genes organized into a single functional unit. These structures have been classified into major types based on a combination of gene content and shared homology. Hitherto, two different grouping schemes and nomenclatures have been used by investigators, as described in recent reviews. In one classification there are three major types, referred to as types F, P, and I, and these types associate with model conjugation systems described for plasmids F, RP4, and R64, respectively. In the other classification, types F and P are grouped together as type IVA and type I is type IVB. A third group in this classification is composed of "other" T4SS representatives (9, 10, 13, 32).

Hitherto, a major unresolved feature of genomic islands was the mechanism by which they are transferred between bacteria. The most widely held view is that genomic islands represent mobile elements, such as phage or conjugative plasmids that have either lysogenized or cointegrated with the chromosome, and that their transfer functions have become degenerate (17). This hypothesis has been favored by a number of investigators; however, observations of a family of syntenic genomic islands with deep evolutionary relationships have challenged this hypothesis based on findings for two members of this family, ICEHin1056 and the clc element. It is recognized that both ICEHin1056 and the clc element are capable of integration into the chromosome of the host, excision, and self-transfer to a new host and reintegration (15, 16, 49).

Preliminary analysis of in silico data acquired from sequencing projects suggested that there is a highly conserved module of genes that is responsible for horizontal transfer of these genomic islands; however, no functional analysis of the components of this putative transfer module was performed, and there was no in-depth description. A better understanding of genes involved in conjugative transfer and their relationship to well-characterized conjugative systems should provide a better understanding of how at least one family of genomic islands is mobilized in bacteria and may suggest that many more genomic islands than currently recognized contain genes for self-mobilization.

Here we describe identification and functional analysis of a cluster of genes encoding an uncharacterized T4SS present in ICEHin1056, an example of a diverse family of genomic islands. Within this gene cluster are homologues of genes in previously defined T4SSs. We show that this novel T4SS is responsible for formation of the conjugative pilus and the resulting conjugative transfer of ICEHin1056. In this study we not only identified a new lineage of T4SSs but also demonstrated that a highly evolved and conserved cluster of genes is involved in the mobilization of genomic islands.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Bacterial strains, plasmids, and growth conditions. All bacterial strains and plasmids used in this study are listed in Table 1. In most cases H. influenzae was grown on HIB medium (Columbia agar containing 15 µg/ml NAD and 15 µg/ml hemin). When required, this medium was supplemented with kanamycin (10 µg/ml), tetracycline (2 µg/ml), or ampicillin (4 µg/ml). All Haemophilus plate cultures were grown for 24 to 48 h at 37°C in an atmosphere containing 5% CO₂. Liquid cultures of H. influenzae were grown in brain heart infusion broth (BHI) supplemented with NAD (10 µg/ml), hemin (15 µg/ml), and, when necessary, antibiotics at the concentrations described above and incubated at 200 rpm on a rotatory shaker at 37°C. Luria-Bertani broth was routinely used for growing Escherichia coli strains. When appropriate, Luria-Bertani medium was supplemented with ampicillin (50 µg/ml), kanamycin (50 µg/ml), or gentamicin (5 µg/ml).

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TABLE 1. Bacterial strains and plasmids used in this work

PCR amplification and recombinant DNA methodology. Restriction endonucleases and DNA-modifying enzymes were obtained from New England Biolabs and were used according to the manufacturer's instructions. Recombinant DNA techniques and plasmid DNA preparation were performed as described by Sambrook et al. (45). Oligonucleotide primers were synthesized by Operon. Standard PCR amplifications were performed in 10-µl mixtures using Taq DNA polymerase or ProofStart DNA polymerase (QIAGEN) according to the supplier's instructions. Four-kilobase DNA fragments comprising the putative transfer region of ICEHin1056 were cloned into pGEM-T Easy (Promega). Recombinant plasmids were confirmed by PCR amplification and restriction endonuclease digestion. Genes of the putative type IV secretion system (tfc1, tfc2, tfc4, tfc6, tfc12, tfc14, tfc16, tfc22, tfc23, and tfc24) were inactivated by inserting a kanamycin resistance cassette and the H. influenzae DNA uptake signal sequence (47). These genes were chosen primarily because they exhibit homology to genes of other T4SSs; however, one of these genes, tfc19, a homologue of traG, lacked suitable restriction sites for insertional mutagenesis. An additional four genes in the module were inactivated because of well-placed unique restriction sites.

Generation of H. influenzae mutant strains. For construction of mutant strains by reciprocal recombination, 2 to 3 µg of the appropriate linearized plasmid DNA construct was used to transform H. influenzae strain Rd11 (Table 1), and transformants were selected with kanamycin (27). All transformants were checked by reculturing on HIB medium containing kanamycin and were subsequently confirmed to be mutants by PCR amplification. The PCR results were consistent with the hypothesis that only one copy of ICEHin1056 was present in each bacterium.

Conjugal transfer of the ICEHin1056 genomic island. The transfer efficiency of ICEHin1056 was determined using a modification of the method of Stuy (48). Donor or recipient cells were grown for 48 h on HIB agar, and approximately 10⁸ cells were scraped off each plate and resuspended in 1 ml of BHI broth. Ten microliters of the suspension of donor cells and 100 µl of the suspension of recipient cells were gently mixed to obtain a ratio of donor cells to recipient cells of 1:10. Subsequently, this mixture was spread in the center of antibiotic-free HIB agar plates. The inoculum was allowed to dry prior to incubation for 6 h. The cells were then harvested by flooding the plate with 1 ml of BHI broth and resuspending them using a spreading paddle. Serial dilutions were plated to determine the viable counts and to determine the number of transconjugants, donors, and recipients using agar containing the appropriate selective antibiotic. Recipients and transconjugants were purified on agar plates containing tetracycline (2 µg/ml) and on agar plates containing kanamycin (10 µg/ml) plus tetracycline (2 µg/ml), respectively. Experiments were carried out in triplicate, and the mean and standard error for each strain were calculated.

Electron microscopy. To examine H. influenzae strains for the presence of the type IV secretion-dependent pilus, bacteria were grown overnight and then allowed to conjugate on antibiotic-free HIB agar plates for 6 h. The conjugating cells were harvested by flooding each plate with 1 ml of distilled water and subsequently resuspended them using a spreading paddle. A copper grid coated with Formvar and carbon was floated in the resulting suspension for 2 min. Excess fluid from the copper grid was removed prior to negative staining with 1% methyl tungstate. The cells were examined to determine the presence of a pilus by transmission electron microscopy.

Reverse transcriptase PCR (RT-PCR). Following growth for 12 h on solid medium and growth to an optical density at 600 nm of 0.6 in liquid medium, total RNA was prepared from harvested bacteria using the SV total RNA isolation system (Promega); the strains used were H. influenzae strains Rd and Rd11, as well as Rd11tfc1, Rd11tfc2, Rd11tfc4, Rd11tfc6, Rd11tfc12, Rd11tfc14, Rd11tfc16, Rd11tfc22, Rd11tfc23, Rd11tfc24, which represented mutants with mutations in the putative T4SS. Following RNA isolation, the yield was quantified spectrophotometrically. Subsequently, 550 ng of total RNA was reverse transcribed to produce cDNA using Superscript II reverse transcriptase (Invitrogen) and random primers (Promega). Coamplification of the cDNA transcripts for genes of the putative T4SS with an internal control allowed comparison of the levels of expression in the different RNA samples. PCR amplification of the cDNA of the putative T4SS genes was performed with oligonucleotides and probes listed in Table 2. Primers specific for the constitutively expressed housekeeping gene encoding beta-lactamase were also included in each PCR amplification as an internal control. To control for chromosomal DNA contamination, RNA samples were also directly used for PCR amplification.

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TABLE 2. Oligonucleotide primers and TaqMan probes used in RT-PCR analysis of gene expression

Quantification of the tfc16 and tfc17 transcripts in strains Rd11 and Rd11tfc24 was performed using an ABI Prism 7000 sequence detection system (Applied Biosystems) according to the manufacturer's recommendations. Probes were labeled with 6-carboxyfluorescein. Plasmid standards were obtained by preparing serial 10-fold dilutions in 100 µl (final volume) of nuclease-free water such that 5 µl of standard added to a 50-µl real-time quantitative PCR mixture (1x TaqMan Universal PCR Master Mix [Applied Biosystems], each primer [Applied Biosystems] at a concentration of 500 nM, 100 nM TaqMan 6-carboxyfluorescein-labeled probe [Applied Biosystems]) contained from 3 x 10⁵ to 3 x 10¹ copies. The reporter fluorescence was normalized to the fluorescence of the passive reference dye ROX. Amplification data, expressed as increases in reporter fluorescence, were collected in real time and were analyzed using the sequence detection system. Experiments were repeated in triplicate with two batches of cDNA made from RNA samples obtained from two independent preparations.

DNA sequence analysis. DNA sequence similarity searches using the BLASTN and BLASTX algorithms (1) and position-specific iterated BLAST (PSI-BLAST) (2) were performed by interrogating the National Center for Biotechnology Information (NCBI) website (http://ncbi.nlm.nih.gov/). The Smith-Waterman algorithm, using the Blosum65 score matrix, a gap opening penalty of 11, and a gap extension penalty of 1, was used to align low-homology open reading frames (ORFs) with the ORFs of known T4SSs and to compute P values for homology (29), using a Bonferroni correction to account for multiple testing. The Artemis comparison tool (ACT) (42) was used to visually compare T4SSs of various bacteria pairwise for homology by using the TBLASTX algorithm. The presence and locations of signal peptide cleavage sites in amino acid sequences were predicted with SignalP 3.0 Server (http://www.cbs.dtu.dk/services/SignalP/), using a combination of several artificial neural networks and hidden Markov models (6).

Evolutionary analysis. The evolutionary relationships of a set of genomic islands that carry putative T4SSs were investigated using three representative essential genes (tfc6, tfc14, and tfc16). Homologues of only these genes were present in all the genomic islands represented in the comparative analysis. It has been claimed elsewhere that tfc14 and tfc16 encode the archetypal T4SS proteins (32). The relationships were estimated using ClustalX based on either the amino acid sequences encoded by each gene individually or the amino acid sequences encoded by the concatenated sequences of tfc14 and tfc16. For these analyses, the following settings were used: correction for multiple substitutions and exclusion of positions with gaps. The output from the phylogenetic analysis was visualized with the TreeView software (38).

Top Abstract Introduction Materials and Methods Results Discussion References

 
Identification of the conserved type IV secretion system. Genomic island ICEHin1056, which is an archetype of a genomic island that accounts for pandemic spread of antibiotic resistance among H. influenzae, is 59.4 kb long and consists of 64 ORFs. Based on preliminary bioinformatic analyses, 24 of the 64 ORFs of ICEHin1056, designated tfc1 to tfc24 (Fig. 1a), were thought to encode an uncharacterized T4SS. Seven of the 24 gene products of the ICEHin1056 putative T4SS have transmembrane helices, suggesting membrane-bound localization of the relevant proteins (Fig. 1a). Moreover, sequence analysis with SignalP 3.0 Server (6) revealed that 10 of the 24 genes of this gene cluster have signal peptide sequences that are typical for genes involved in T4SSs (Fig. 1a) (34). Characteristically, T4SSs are encoded by many genes expressing transmembrane domains and signal peptide sequences (32). DNA sequence similarity searches using the BLASTN and BLASTX algorithms (1) and position-specific iterated BLAST (2) together with the Smith-Waterman algorithm (29) were employed to identify components of this putative T4SS. This bioinformatic analysis revealed homology between tfc1 and the known T4SS gene traX (E = 1 x 10^–60 in PSI-BLAST iteration 2), encoding a protein responsible for acetylation of F plasmid prepilin (35). Moreover, tfc6 was homologous to traD (E = 2 x 10^–166 in PSI-BLAST iteration 1), a gene encoding a T4SS conjugative coupling protein involved in synchronization of mating pair formation with DNA transfer and "pumping" of DNA into recipient cells (22, 25). tfc14, tfc16, and tfc19 are traB (E = 9 x 10^–06 in PSI-BLAST iteration 2), traC/virB4 (E < 3 x 10^–159 in PSI-BLAST iteration 1), and traG (E = 2 x 10^–42 in PSI-BLAST iteration 3) homologues, respectively, all of which are involved in pilus assembly (3, 32, 40). tfc2 and tfc22 exhibit homology with pilL (E = 4 x 10^–19 in PSI-BLAST iteration 1) and pilT (E = 1 x 10^–21 in PSI-BLAST iteration 1), respectively, whose products are required for type IV secretion pilus biogenesis (43, 44). Altogether, these in silico findings (Fig. 1a) suggest that the gene cluster which we investigated encodes components of a T4SS. Moreover, this gene cluster has been recognized in related genomic islands of other bacterial species, including Legionella pneumophila, Pseudomonas aeruginosa, Pseudomonas fluorescens, Photorabdus luminescens, Erwinia carotovora, and Salmonella enterica serovar Typhi.

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FIG. 1. (a) Schematic diagram of the ICEHin1056 genomic island and localization of the putative type IV secretion system (red). This T4SS is encoded by 24 genes, 7 of which exhibit homology to known T4SS components (components a to g). Seven of 24 gene products of the ICEHin1056 putative T4SS have transmembrane helices (indicated by plus signs), suggesting membrane-bound localization of the relevant proteins. Moreover, 10 of 24 genes of this gene cluster have signal peptide sequences (indicated by plus signs) that are typical for genes involved in T4SSs. a, TraX homologue (F plasmid); b, PilL homologue (plasmid pKLC102); c, TraD homologue (F plasmid); d, TraB homologue (F plasmid); e, TraC/VirB4 homologue (F plasmid); f, TraG homologue (F plasmid); g, PilT homologue (L. pneumophila str. Philadelphia 1). (b) Transcriptional mapping by RT-PCR of the ICEHin1056 T4SS gene cluster. The T4SS gene cluster is expressed in the form of two polycistronic transcripts. Cotranscription of neighboring genes is indicated by solid boxes. Neighboring genes which are not transcribed together are indicated by open boxes. (c) Gel showing that no mRNA transcripts were detected between tfc19 and tfc20 and between the first gene (tfc1) and the last gene (tfc24) of the ICEHin1056 T4SS and the neighboring genes in the rest of the ICEHin1056 sequence (ORF ICEHin1056). Lane 1, ORF ICEHin1056-tfc1 transcript; lane 2, tfc1-tfc2 transcript; lane 3, tfc2-tfc3 transcript; lane 4, tfc3-tfc4-tfc5-tfc6 transcript; lane 5, tfc6-tfc7-tfc8 transcript; lane 6, tfc8-tfc9 transcript; lane 7, tfc9-tfc10-tfc11 transcript; lane 8, tfc10-tfc11-tfc12-tfc13 transcript; lane 9, tfc13-tfc14 transcript; lane 10, tfc14-tfc15 transcript; lane 11, tfc15-tfc16 transcript; lane 12, tfc16-tfc17-tfc18-tfc19 transcript; lane 13, tfc19-tfc20 transcript; lane 14, tfc20-tfc21 transcript; lane 15, tfc21-tfc22 transcript; lane 16, tfc22-tfc23-tfc24 transcript; lane 17, tfc24-ORF ICEHin1056 transcript.

ICEHin1056 type IV secretion system is evolutionarily distant from previously described T4SSs. Work over the past few years has led to classification of three distinct clusters of T4SSs: F-like (21, 22, 32, 33, 37), P-like (12, 13), and I-like (31, 46, 50). Relationships within and among clusters are based mainly on three genes, identified as either tfc6, tfc14, and tfc16 or traD, traB, and traC/virB4, respectively (named for the E. coli F plasmid T4SS). The tfc14 and tfc16 genes encode the quintessential T4SS proteins (32) and as such are ideal for comparative amino acid alignment analysis. The trees for these three genes showed different patterns of clustering across genomic islands, but substantial compatibility between trees was evident based on homologies to tfc14 and tfc16 (Fig. 2a to c). Combining the data for both sets of homologues of tfc14 and tfc16 produced a tree with strong bootstrap support for four major clusters (Fig. 3). The fourth type of T4SS is carried by a wide variety of genomic islands, including the clc element, pKLC102, PAPI, and Salmonella pathogenicity island 7, as well as other genomic islands derived from the following bacterial strains: L. pneumophila, P. aeruginosa, P. fluorescens, E. carotovora, and S. enterica serovar Typhi (Fig. 3).

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FIG. 2. Phylogenetic trees based on ClustalX analysis, showing the evolutionary relationships between amino acid sequences encoded by tfc14 (a), tfc16 (b), and tfc6 (c) of the ICEHin1056 T4SS and homologues from a set including the previously described T4SSs and homologues identified by bioinformatic analysis from a wide range of proteobacteria. The topology of the trees resulting from alignment of amino acid sequences encoded by tfc6, tfc14, and tfc16 and their homologues revealed that the topology was conserved only for tfc14 and tfc16. S Typhi, Salmonella enterica serovar Typhi; V cholerae, Vibrio cholerae; P rettgeri, Proteus rettgeri; P vulgaris, Proteus vulgaris; N aromaticivorans, Neisseria aromaticivorans; N gonorrhoeae, Neisseria gonorrhoeae; A rhizogenes, Agrobacterium rhizogenes.

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FIG. 3. Phylogenetic tree based on ClustalX analysis, showing the evolutionary relationship between concatenated amino acid sequences of tfc14 and tfc16 of the ICEHin1056 T4SS and the previously described T4SSs. The ICEHin1056 T4SS does not belong to any of the previously described groups and is evolutionarily distant. Moreover, syntenic T4SSs located on a broad spectrum of genomic islands present in a wide variety of bacterial species belong to this evolutionarily distant group. For an explanation of abbreviations see the legend to Fig. 2.

Role of the ICEHin1056 type IV secretion system in pilus biogenesis. T4SSs are multisubunit cell envelope-spanning structures comprising a secretion channel and often a pilus or other surface filament or protein (13, 19, 32, 41, 51, 52). Despite the presence of a vestigial pilus gene cluster in the genome, H. influenzae strain Rd produces neither fimbriae nor type IV pili (5, 20). However, electron microscopic examination of strain Rd harboring genomic island ICEHin1056 (strain Rd11) revealed long thin pili (Fig. 4a to d), which is consistent with the hypothesis that the genes responsible for formation of pili are located on ICEHin1056. Only a single pilus approximately 7 to 7.5 nm in diameter and 1.5 µm long was attached to bacteria (Fig. 4a). In order to demonstrate the essential role of this putative T4SS for the formation of pili, 10 genes in this gene cluster, tfc1, tfc2, tfc4, tfc6, tfc12, tfc14, tfc16, tfc22, tfc23, and tfc24, were disrupted by insertion of a kanamycin resistance cassette, and mutant strains were examined for expression of pili by electron microscopy. These genes were chosen primarily because they exhibit homology to genes of other T4SSs or because of well-placed unique restriction sites. As shown in Table 3, disruption of these genes had a dramatic effect on the abilities of the mutants tested to produce pili. Pili were not detected in 7 of the 10 mutants analyzed, confirming the crucial role of this gene module in T4SS-dependent pilus expression. Not all genes associated with known T4SSs are involved in pilus biogenesis (13, 33). Disruption of the remaining three genes, tfc1, tfc6 and tfc24, did not diminish the production of pili (Fig. 4b to d and Table 3). The pilus-positive phenotype of strains Rd11tfc1 and Rd11tfc24 provided an explanation for the observed slight changes in the conjugation frequencies described below and suggest that tfc1 and tfc24 do not play a significant role in the formation of the conjugative type IV pili under our in vitro conditions. By contrast, despite its massive reduction in conjugative transfer, the mutant with a mutation in the tfc6 gene, encoding the conjugative coupling protein, produced pili (Table 3), confirming the hypothesis described by Panicker and Minkley (39) and verified by several other research groups that conjugative coupling proteins are essential for conjugation but are not required for T4SS pilus synthesis. However, most of the mutants examined were not able to express pili, revealing the essential role of this gene module in the expression of type IV secretion-dependent pili. None of the mutants has been complemented in trans as there is not a complementation system in H. influenzae that is easy to use; however, our experimental data show that the mutations had no polar effects on at least two of the downstream genes (tfc6 and tfc24 mutants produced pili in spite of pilus-negative phenotypes of upstream genes).

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FIG. 4. Transmission electron micrographs of negatively stained samples, showing the presence of pili dependent on the T4SS (arrows) in H. influenzae strain Rd harboring ICEHin1056 (a), the tfc1 mutant (b), the tfc6 mutant (c), and the tfc24 mutant (d), B, bacterium. Bars = 100 nm.

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TABLE 3. Conjugation frequencies and pilus formation for mutants with mutations in the ICEHin1056 type IV secretion system

ICEHin1056 type IV secretion system is important for conjugation. Conjugation systems, assembled by a subfamily of the T4SSs, are usually used by bacteria in the process of conjugative transfer of DNA from donor cells to recipient cells (11). Genomic island ICEHin1056 in the parent strain was transferred from the donor to the recipient strain of H. influenzae at a frequency of 3 x 10^–2. To obtain evidence for the function of the ICEHin1056 T4SS, all T4SS mutant strains were tested to determine their abilities to transfer the genomic island by conjugation to the same recipient. As shown in Table 3, the conjugation frequencies of the mutants were significantly lower than the conjugation frequency of the parent strain and ranged from 6 x 10^–3 to 1 x 10^–7. In agreement with our expectations, the lowest conjugation frequency, 1 x 10^–7, was observed for the mutant with the disrupted tfc6 gene, encoding a putative conjugative coupling protein (8). Disruption of three genes, tfc1, tfc4, and tfc24, reduced the conjugal transfer frequencies to approximately 10^–3, whereas disruption of the remaining seven genes, tfc2, tfc6, tfc12, tfc14, tfc16, tfc22, and tfc23, led to reduction of the conjugal transfer frequencies to approximately 10^–7. In summary, the abilities of the majority of the mutants tested to conjugate were critically impaired, and the frequencies of transfer were reduced up to 100,000-fold compared to the frequencies of transfer observed for the parent strain, providing functional evidence of the importance of the T4SS for conjugal transfer of ICEHin1056 DNA; hence, we used the designation tfc (type four conjugation) for all components of this gene cluster.

Transcriptional analysis of the ICEHin1056 type IV secretion system. RT-PCR analysis of gene expression revealed that the ICEHin1056 T4SS gene cluster is expressed in the form of polycistronic transcripts (Fig. 1b and c). No mRNA transcripts were detected between tfc19 and tfc20 and between the first gene (tfc1) and the last gene (tfc24) of the ICEHin1056 type IV secretion cluster analyzed and the neighboring genes in the rest of the ICEHin1056 sequence (Fig. 1b and c). As Fig. 1b and c show, the RT-PCR analysis results suggest that the module consists of two transcripts. Transcriptional analysis by RT-PCR (Fig. 1b and c) revealed amplicons for the gene downstream of the gene interrupted by insertion of an antibiotic resistance cassette in each of the 10 T4SS mutant strains tested (data not shown). This suggests that there are no polar effects on genes downstream of insertional mutations.

Interestingly, RT-PCR analysis of gene expression of the ICEHin1056 T4SS indicated that there were no substantial changes in the pattern of expression in the mutants tested compared to the wild-type pattern, except for mutant tfc24. As shown in Fig. 5a, mutation of tfc24 greatly reduced expression of many of the type IV secretion genes. Quantification of tfc16 and tfc17 mRNA transcripts in strains Rd11 and Rd11tfc24 using the ABI Prism 7000 sequence detection system revealed that the number of copies of the tfc16 gene transcript was reduced from 530 x 10² in strain Rd11 to 263 x 10² in mutant Rd11tfc24 (Fig. 5b). Moreover, the number of copies of the tfc17 gene transcript was reduced from 537 x 10² in strain Rd11 to 411 x 10² in mutant Rd11tfc24 (Fig. 5b). The tfc24 product exhibits strong homology with conserved proteins having unknown functions found in a wide variety of bacterial species, including Haemophilus somnus, Haemophilus ducreyi, P. aeruginosa, P. fluorescens, Pseudomonas syringae, P. luminescens, E. carotovora, S. enterica serovar Typhi, and Yersinia pseudotuberculosis, and it contains a protein kinase C phosphorylation site. This suggests that tfc24 might encode a protein which plays a crucial role in regulation of the T4SS, whose function might be conserved between species. The role of this gene in regulation remains to be elucidated.

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FIG. 5. RT-PCR showing transcription levels of genes of the ICEHin1056 T4SS in H. influenzae strain Rd harboring ICEHin1056 (Rd11) and in the tfc24 mutant (Rd11tfc24). (a) Expression of the majority of the genes of the T4SS examined is downregulated in the tfc24 mutant. The results also demonstrate that there is cotranscription of T4SS genes. Lane 1, tfc1 transcript; lane 2, tfc3-tfc4-tfc5-tfc6 transcript; lane 3, tfc6 transcript; lane 4, tfc6-tfc7-tfc8 transcript; lane 5, tfc9-tfc10-tfc11 transcript; lane 6, tfc12-tfc13 transcript; lane 7, tfc13 transcript; lane 8, tfc14 transcript; lane 9, tfc14-tfc15 transcript; lane 10, tfc16 transcript; lane 11, tfc17-tfc18-tfc19 transcript; lane 12, tfc22 transcript; lane 13, tfc23-tfc24 transcript. (b) Quantification of the tfc16 and tfc17 mRNA transcripts in strains Rd11 and Rd11tfc24 using the ABI Prism 7000 sequence detection system. Interpolating from the standard curve, the number of copies of the tfc16 gene transcript was reduced from 530 x 102 in strain Rd11 to 263 x 102 in mutant Rd11tfc24 and the number of copies of the tfc17 gene transcript was reduced from 537 x 102 in strain Rd11 to 411 x 102 in mutant Rd11tfc24.

Top Abstract Introduction Materials and Methods Results Discussion References

 
A set of genes present in ICEHin1056, a vector of antibiotic resistance in H. influenzae, was characterized, and we concluded that this vector represents a new family of T4SSs involved in DNA translocation among gram-negative bacteria. This interpretation was based on bioinformatic, evolutionary, and functional analyses. First, in the bioinformatic analysis, both protein motifs typical of T4SS gene clusters were found, and homologies to genes known to be associated with T4SSs were identified. Second, the evolutionary analysis of the two archetypal T4SS proteins revealed high levels of similarity between amino acid sequences for the three previously identified T4SS types plus a novel fourth type. Third, mutagenesis of genes indicated that the genes are involved in pilus formation and the resulting transfer of DNA by conjugation. Lastly, a transcriptional analysis suggested that this gene cluster is probably part of a regulated operon typical of other T4SSs. The putative origin of transfer has been localized between ORFs 60 and 61 of ICEHin1056, a position that is distinct from the position of the putative T4SS module investigated.

The genes that we analyzed are members of a set of core genes found in a diverse family of syntenic genomic islands (36). Well-recognized members of this family of genomic islands are widely distributed among proteobacteria, and additional variants have been identified among completed genome sequences present in NCBI databases.

Analysis of mRNA by RT-PCR suggested that transcription of the T4SS genes is polycistronic. The gene disruptions in the T4SS mutants described here have not been complemented in trans as no reliable complementation system is available for H. influenzae; however, our experimental data show that the mutations did not have polar effects on the downstream genes. Both tfc6 and tfc24 mutants produce pili in spite of the pilus-negative phenotype of upstream genes. Similarly, the conjugation frequencies of tfc4 and tfc24 mutants are only slightly reduced compared to those of mutants with mutations in upstream genes. Furthermore, transcriptional analysis suggested that there was no clear polar effect of gene disruption on downstream transcripts. This might be explained by the fact that the kanamycin resistance cassette used harbors an endogenous promoter which, when oriented in the same direction as the target gene, does not interfere with downstream gene expression.

Recent reviews of T4SSs have described the relationship between the recognized T4SSs (9, 13, 32). A simple consensus view is that T4SSs share two features: they encode bacterial protein and nucleoprotein export systems, and members of each type exhibit evidence of common ancestry. This definition is broad, and its interpretation is flexible; therefore, it is not surprising that the T4SS gene clusters described previously are diverse in a number of respects. They vary in gene content, the number of homologues that they share, and the gene order. This heterogeneity is a challenge to a simple classification. Therefore, it is not surprising that two approaches have been used for classification. Both approaches were founded on a combination of the genetic organization of the cluster and the evolutionary relationships among homologous genes. The first classification recognized three types based on the incompatibility group of the representative and archetypal DNA conjugative plasmid, namely, type F for IncF (plasmid F), type P for IncP (plasmid RP4), and type I for IncI (plasmid R64) (32). There is greater conservation of gene content and order within each type than there is between types. Alignment of amino acid sequences shared by proteins encoded by homologous genes supports the subdivision into these three groups. The alternative classification recognizes two major groups, type IVA and type IVB, and a third, "other" group (9, 10, 13). Type IVA includes the F and P types referred to above. Type IVB consists of type I, whose component modules are considered to be highly divergent from those of members of both type F and type P. The third group, labeled the "other" group, has not been well characterized.

The T4SS gene cluster reported here is distinct from previously recognized types whichever typing scheme is used. It represents a fourth lineage with a genetic distance as great as that between the F, P, and I lineages. The component genes of this module exhibit homology to a limited number of genes in the other lineages; only four homologues, tfc6 (traD), tfc14 (traB), tfc16 (traC/virB4), and tfc19 (traG) (Fig. 1a), are shared with the 12-gene VirB/D4 system found in the Ti plasmid of Agrobacterium tumefaciens and with the more numerous genes contributing to the T4SS gene clusters of the F plasmid and plasmid R64. Thus, using the alternative classification system, this new T4SS would be classified as a member of neither type IVA or type IVB but as a member of the "other" group. Despite the involvement in DNA conjugation, there is no recognized incompatibility group for the elements containing the new T4SS; consequently, it does not readily fit into the typing scheme consisting of types F, P, and I. We designated this lineage the GI type, as this T4SS is associated exclusively with genomic islands, as explained further below. The GI type T4SS gene cluster is a core component of a highly diverse family of syntenic genomic islands that presumably evolved in proteobacteria. Remarkably, the G+C contents of the GI type gene clusters found in this family of genomic islands range from 40% to approximately 70%, indicating that there is substantial divergence and evolutionary separation; however, the gene content and order have remained largely intact (36). This suggests that there is a particular fitness advantage to this coherent set of genes acting together, and intriguingly, so far these genes appear to be uniquely associated with a specific family of genomic islands.

The observations reported here describing the GI type of T4SS module from the archetypal genomic island ICEHin1056 provide direct evidence of how genomic islands may conjugally transfer. Hitherto, there was doubt about how such widely distributed structures could rapidly mobilize and spread through bacterial populations. Here we present evidence that one related, diverse, and widely distributed family of genomic islands possesses the genetic wherewithal to conjugate. Only two examples of this family of genomic islands have been shown to be conjugally transferable, namely, ICEHin1056 and the clc element (23, 36). It can be inferred that other members of this family of genomic islands mobilize intact from the distribution of the following genomic islands among their host bacteria: Salmonella pathogenicity island 7 (S. enterica serovar Typhi) (7), the Yersinia adhesion pathogenicity island (Y. pseudotuberculosis) (14), and PAP1 and pKLC102-like genomic islands (P. aeruginosa) (26, 30). The presence of a highly evolved and efficient conjugation system for mobilizing genomic islands provides a better explanation of how one family of genomic islands is able to propagate and efficiently enable bacterial populations to adapt to rapidly changing environments. What remains to be seen is whether the many diverse genomic islands not related to ICEHin1056 have dedicated conjugation systems that enable them to propagate through conjugation. Only recently have bioinformatic techniques become powerful enough to recognize and make predictions allowing recognition of the ICEHin1056 family of genomic islands and their related T4SS gene clusters. It is quite plausible that other distinct families of genomic islands will be identified and that new and divergent lineages (families) of T4SS gene clusters will be recognized.

Potentially, 24 genes were found in the T4SS module of ICEHin1056, and 7 of these genes are homologous to genes directly or indirectly associated with other T4SS gene clusters. Four of these seven genes are homologues to VirB/D4 genes found in the Ti plasmid of A. tumefaciens, a gene cluster considered by many workers to be the minimal essential set of T4SS genes; therefore, there are at least 18 genes with no homology to any of the remaining genes in the other type F, P, and I T4SS lineages, as determined by currently available techniques. Similarly, there are genes in types F, P, and I that are unique to each type, and the number may be more than 10 for type I and for type F. This heterogeneity between types for genes forming a T4SS module is consistent with the hypothesis that these genes were assembled through recruitment by a mixture of convergent (horizontal transfer) and divergent evolutionary processes. Curiously, within each type, there is marked preservation of coherence in each T4SS gene cluster. Once assembled, despite opportunities for recombination and reordering of component genes, the cluster apparently remains coherent due to a particular fitness advantage for a coordinated set of genes. Intriguingly, at least two genes, traC/virB4 and traB, appear to be sufficient to define membership in their respective lineages and may be essential for a cluster to function as a T4SS export system. If these two genes provide the key signature identifying other poorly related T4SSs among genomes and genomic islands in particular, searching for homologues of these genes may be the way to identify other distinct T4SS lineages.

 
We cordially thank E. Richard Moxon and Sarah L. Turner for critical reviews and comments on the manuscript and Kate Dingle and Gaynor Jenkins for their technical assistance with RT-PCR expression analysis.

This work was supported by a grant from the Medical Research Council. D.J.P.F. was supported by an equipment grant from the Wellcome Trust.

 
* Corresponding author. Mailing address: Clinical Microbiology and Infectious Diseases, Nuffield Department of Clinical Laboratory Sciences, University of Oxford, Headington, OX3 9DU Oxford, United Kingdom. Phone: (0044) 01865 222194. Fax: (0044) 01865 222195. E-mail: mario.juhas{at}ndcls.ox.ac.uk.

Published ahead of print on 22 November 2006.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Journal of Bacteriology, February 2007, p. 761-771, Vol. 189, No. 3
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