Distribution of Centromere-Like parS Sites in Bacteria: Insights from Comparative Genomics

Programs and search parameters used.Blast comparisons were conducted using BLASTN and BLASTP 2.0MP-WashU (1). Search parameters B (the maximum number of database sequences for which any alignments will be reported) and V (the maximum number of one-line descriptions of significant database sequences reported) were set to 10,000. Unless otherwise noted, all other search parameters were set to default values. Motif searches were conducted using Patser v3e.1 (54) and RNAMotif v3.0.4 (41). For Patser searches, the a priori nucleotide probabilities used to convert the alignment matrix to a weight matrix were set to 1 for all four nucleotides. Scrambled matrices were created by shuffling the columns in each half-site of the parS consensus matrix symmetrically so that the resulting matrices maintained the same palindromic structure, length, and overall base frequencies as the parS matrix but corresponded to different primary consensus sequences.

Sequence databases.Genome sequence files (.fna extensions), protein sequence files (.faa extensions), and open reading frame (ORF) annotation files (.ptt extensions) were obtained from the NCBI ftp database (ftp://ftp.ncbi.nih.gov/genomes/Bacteria/ ).

oriC predictions.In circular bacterial chromosomes, the leading strand often contains more G than C nucleotides, and the origin of replication can be identified using this G/C skew (40). In many but not all published genomes, the annotation ±1 has been assigned based on the location of the GC skew minimum (and thus likely corresponds to oriC). Another feature common to the oriC regions of diverse species is their proximity to genes encoding homologues of the replication initiator protein DnaA and of the glucose-inhibited division protein GidA (9, 18, 23, 27, 45, 48, 63). For our analyses we defined the putative oriCs as follows.

(i) Circular primary chromosomes.If the annotated +1 was within 2% of the genome size from both the putative DnaA- and GidA-encoding genes (identified by homology to the DnaA and GidA of V. cholerae, Escherichia coli, and B. subtilis), the putative oriC was assigned at the putative +1. Alternatively, if the annotated +1 was farther than 2% of the genome size from either of the putative DnaA- and GidA-encoding genes, the GC minimum was determined using GenSkew (http://mips.gsf.de/services/analysis/genskew ) and the putative oriC was assigned based on this minimum.

(ii) Linear primary chromosomes.The oriC was assigned at a position directly upstream of the dnaA gene.

(iii) Secondary chromosomes.No DnaA or GidA homologues were identified on secondary chromosomes. Thus, the oriCs of secondary chromosomes were assigned based on the GC skew minima identified by GenSkew.

Construction of parS2/IR consensus matrices.The V. cholerae chromosome II parS site consensus matrix was created using the nine V. cholerae parS2 sites identified by Yamaichi et al. (60). The B. cenocepacia chromosome II and chromosome III parS site consensus matrices were created using the six and the four B. cenocepacia parS2 and parS3 sites, respectively, that were identified by Dubarry et al. (13). To construct the Ralstonia parS2 consensus matrix, RNAMotif was used to search the sequence of chromosome II of Ralstonia eutropha H16 for perfect palindromes corresponding to the motif 5′-TTN(4)CGN(4)AA-3′. This motif was based on the putative parS site identified by Dubarry et al. on pGMI1000MP of R. solanacearum. The six R. eutropha sites identified in this search were incorporated into the R. eutropha parS2 consensus matrix. To construct the consensus matrix for the secondary chromosome IR (parS) sites of Brucella suis, RNAMotif was used to search for any 7-bp inverted repeat flanking two central bases in the 6-kb region of the Brucella suis chromosome II that is centered at the repABC operon. This search led to the identification of one site that, similar to other IR sites, contained a central GC motif and was located between the repA and repB genes. Patser was then used to search for other sites similar to this sequence in the entire chromosome, leading to the identification of another palindromic sequence with a central GC directly upstream of the repA gene. These two sites were incorporated into the consensus matrix.

Informatic approach used to search for putative parS sites in prokaryotic replicons.To search for putative parS sites in all 706 sequenced prokaryotic chromosomes, plasmids, and extrachromosomal elements in the NCBI database, we generated a parS consensus matrix using previously characterized chromosomal parS sequences (Fig. 1A). Prior work had demonstrated that consensus matrices are a robust tool for identification of DNA sequence motifs in genomes (50). To help ensure the accuracy of our predictions, only parS sites that have been shown to bind ParB in vivo were used to create the consensus matrix. These included 15 sites from Streptomyces coelicolor (30) and 10 sites from Bacillus subtilis (8, 38). The program Patser (54) was then used to search for sequences corresponding to this matrix in all 706 prokaryotic replicons in the NCBI database (see Table S1 in the supplemental material).

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FIG. 1.

WebLogos of putative parS site consensus sequences. A. Logos representing the 25 parS1 sites used to construct the S. coelicolor and B. subtilis consensus matrix and the 1,030 putative parS sites identified using this matrix. B. Logos representing the putative parS sites identified in secondary chromosomes using the indicated consensus matrices. The shaded boxes highlight conserved motifs.

Patser searches were repeated with different minimum score thresholds, and the optimal minimum score was empirically found to be 15, based on the concordance of the predicted parS sites with the results of previous studies. With the minimum score set to 15, no parS sites were identified in Haemophilus influenzae and E. coli, species that are known not to encode Par proteins. In addition, using this threshold we identified all previously characterized parS sites in Pseudomonas putida and Caulobacter crescentus and on the primary chromosomes of Burkholderia cenocepacia and Vibrio cholerae (13, 22, 42, 43, 60), even though the parS sequences from these organisms were not used in the construction of the parS consensus matrix. Finally, no additional parS sites were identified in these replicons, including several putative parS sites in B. subtilis and V. cholerae that were identified by bioinformatics but shown to not be functional (38, 60).

Although our search parameters appear to have a high degree of specificity, the sensitivity of our search was not perfect, as several previously identified parS sites were not detected. For example, 2 of the 10 verified B. subtilis parS sites were missed in this search. Even though both of these recently described sites (8) were included in the consensus matrix, their sequences diverge significantly from the sequences of the other 23 sites used to generate the matrix, and their identification in a Patser search required a threshold score below 15. These findings suggest that some bona fide parS sites in other replicons whose sequences diverge from those of most other sites have been missed in our predictions.

In Pseudomonas aeruginosa, our search detected 4 of the 10 putative parS sites annotated by Bartosik et al. (4). The four sites we identified diverge least from the B. subtilis parS consensus sequence, are closest to the P. aeruginosa origin of replication, and are the sites that Bartosik et al. suggested were most likely to be functional (4). It is not known if the six putative P. aeruginosa parS sites missed in our search represent functional parS sites. Thus, some of the previously identified parS sites that were missed in our predictions may not correspond to functional parS sites.

The two putative parS sites we identified in Helicobacter pylori were not the same two sites previously reported by Lee et al. (33). The sites we found in our search are the same length as and diverge by only a single base from the B. subtilis consensus sequence; in contrast, the sites identified by Lee et al. are either 1 base longer or shorter than the B. subtilis consensus sequence and diverge from this sequence by >3 bases. Also, the putative parS sites we identified are much closer to the H. pylori origin of replication than the sites found by Lee et al. (33). As discussed below, the proximity of the two parS sites we identified to the putative H. pylori origin of replication supports their validity.

parS sites are found in diverse prokaryotes and only on primary chromosomes.Our search resulted in the identification of 1,030 putative parS sites in 276 (69%) of the 400 sequenced strains in the NCBI database (see Table S2 in the supplemental material). Remarkably, although the parS consensus matrix was derived from two gram-positive species, putative parS sites were found in all branches of the prokaryotic evolutionary tree, including in four strains of archaea (Fig. 2), and their sequences were found to be very well conserved (Fig. 1A). The widespread conservation of this site across diverse species suggests that parS and probably par-based segregation systems arose very early in the evolution of prokaryotes. For the most part, with the exception of two branches of Gammaproteobacteria, organisms lacking parS sites were scattered throughout the prokaryotic evolutionary tree (Fig. 2). Surprisingly, of the 1,030 parS sites identified, only 1 was identified on a plasmid and none were identified on secondary chromosomes, despite the fact that plasmids and secondary chromosomes comprise 7.2% of all sequences in the NCBI database. Thus, the parS sites identified in this search appear to be distinctive features of primary chromosomes.

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FIG. 2.

ParABS profiles generally correlate with phylogenetic relationships. The phlyogenetic groupings are based on a phylogenetic tree available at the STRING website (http://string.embl.de/ ) (55). Phylogenetic phyla and classes are shown by the gray bars. Bact and Chlor correspond to the Bacteroidetes and Chlorobi phyla. The numbers in the S column correspond to the number(s) of parS sites contained by the strains in each branch; red cells denote that all strains in the branch contain a parS site; pink cells denote that only some strains in the branch contain a parS site; empty cells denote that no strain in the branch contains a putative parS site. In columns A and B, cells are shaded if all strains in the branch encode a putative ParA and ParB homologue, respectively; empty cells denote that no strain in the branch encodes a putative ParA or ParB. Asterisks denote that one strain in the branch encodes a ParA or ParB homologue. In column AB->S, light shading denotes branches in which all strains carry all three components within a region of the chromosome corresponding to 10% of its size. The dark shading denotes branches in which, in at least one strain, the distance between ParA or ParB and the nearest parS site is more than 10% of the replicon size. The numbers correspond to the range of these distances among the strains in that branch.

Variability in number but not location of parS sites in different species.As shown in Fig. 3A, there is considerable variability in the number of predicted parS sites in different strains. While the majority of the parS⁺ strains contain one to four putative parS sites, 23 strains belonging to 11 genera are predicted to encode eight or more parS sites. In some cases, all species in the genera (e.g., Streptomyces spp.) have a large number of predicted parS sites; in other cases, there was a large range in the number of parS sites in different species of the same genera (e.g., Lactobacillus spp.) (Fig. 2). The number of putative parS sites predicted in each chromosome did not correlate with chromosome size. For example, the 3-Mbp chromosome of Listeria innocua contains 20 putative parS sites, while the 7-Mbp chromosome of Pseudomonas fluorescens Pf-5 contains only 2. Indeed, among the 400 strains analyzed, the number of parS sites predicted per Mbp of chromosome ranges nearly 42-fold, from 0.2 in Acidobacteria bacterium Ellin345 to 7.5 in Listeria welshimeri.

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FIG. 3.

Distribution of the number of putative partitioning sites encoded per replicon. A. Sites identified on primary chromosomes identified using the S. coelicolor/B. subtilis consensus matrix. B. Sites identified on secondary chromosomes identified using parS2/IR consensus matrices.

As shown in Fig. 4A, the locations of the 1,029 predicted chromosomal parS sites are not distributed randomly throughout the respective genomes. The vast majority of the putative sites were identified within origin-proximal regions of their respective chromosomes. More than 92% of the sites were located in chromosomal regions corresponding to 15% of the respective replicon's size centered at its oriC (referred to below as the “15% oriC region”) (Fig. 4A). The percentage of functional parS sites within the 15% oriC region is likely to be even higher, since many of the 82 sites predicted outside this region are found in species that lack parAB genes. The average distance of the predicted parS sites from the respective oriC was 2.6% of the replicon size. Even in replicons encoding 10 or more parS sites, all sites are clustered within this 15% oriC region.

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FIG. 4.

Distribution of the locations of putative parS sites relative to their respective putative oriCs. Numbers denote the percentage of all putative sites. The top of each circle corresponds to the putative oriC. Each segment represents a region corresponding to 5% of the replicon size. Numbers denote the percentage of all sites located within the segment. A. Sites identified with the S. coelicolor/B. subtilis consensus matrix. B. Sites on secondary chromosomes identified with parS2/IR consensus matrices.

To determine if this positional bias was specific to the parS consensus sequence rather than to 16 bp palindromes in general, we repeated our search using two different “scrambled” matrices representing palindromic motifs of the same length as the parS consensus but corresponding to different primary sequences (see Materials and Methods). Searches using these palindromes yielded only 21 and 91 sites, respectively, suggesting that the parS consensus sequence is greatly overrepresented in prokaryotic chromosomes compared to similar palindromic motifs. Moreover, only 18% of the sites corresponding to the scrambled matrices were found within the 15% oriC regions, suggesting that the proximity of putative chromosomal parS sites to the respective oriCs is specific to the parS consensus sequence. The remarkable positional conservation of parS sites suggests that the function of chromosomal par loci is highly dependent on the proximity of parS to oriC.

Most primary chromosomes that contain parS sites also encode ParA and ParB homologues.To identify ParA and ParB homologues on primary chromosomes, a database of the 1,151,128 annotated proteins in the NCBI database was compared by BLAST to the ParA and ParB proteins encoded by B. subtilis and by the primary chromosome of V. cholerae. The minimum BLAST score was set to 350 to eliminate false positives in species known not to encode true ParA and ParB homologues, such as E. coli and H. influenza. In total, 255 and 282 ParA and ParB homologues, respectively, were identified. All of the putative ParA and ParB homologues identified were encoded on primary chromosomes, an observation consistent with prior studies that suggested chromosomal Par proteins are phylogenetically distinct from Par proteins encoded by secondary chromosomes and plasmids (13, 21). Two hundred forty-five strains were predicted to encode homologues of both ParA and ParB. In most cases, the presence of putative parS sites in a particular strain correlated with the presence of putative ParA and/or ParB homologues. As shown in Fig. 5, 25% of strains do not encode ParA or ParB proteins or contain parS sites (denoted Par0 strains), while 56% of strains appear to contain all three components (denoted Par3 strains). Thus, in 81% of the strains analyzed, the presence or absence of one component of the ParABS system correlates with the presence or absence of the other two components. The apparent absence of Par proteins and parS sites from only 25% of the 400 strains in the NCBI database indicates that Par-mediated processes are pervasive in bacteria. Interestingly, of the 45 archaeal strains in the database, 41 were Par0 and none were ParABS, suggesting that Par-mediated segregation is not conserved in this branch of the prokaryotic evolutionary tree. In addition to the vast majority of strains classified as Par0 or Par3, we identified a significant number of strains that conformed to several other distinct classifications. Thirty-seven strains (9%) were found to encode a ParB homologue and parS sites but not a ParA homologue (ParBS strains). Twenty-one strains (5%) encode ParA and ParB homologues but no predicted parS sites (ParAB strains).

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FIG. 5.

Venn diagram of ParABS profiles in the 400 strains analyzed. The numbers represent the percentage of the strains corresponding to each ParABS profile.

Proximity of parS sites to parAB loci on primary chromosomes.In all previously characterized chromosome-borne par loci, the parA and parB genes are located relatively close to one or more parS sites; furthermore, with the exception of H. pylori (33), the reported par genes and parS sites are usually found in close proximity to oriC. In the majority of Par3 strains we identified, these positional relationships between parA, parB, parS, and oriC are conserved. First, in 197 (88%) of the 224 Par3 strains, the distance between parA or parB and the nearest parS site is less than 10% of the chromosome size (denoted as ParABS strains). Second, in 172 (87%) of the 197 ParABS strains, the putative parA, parB, and nearest parS sites are all located less than 10% of the chromosome size away from the putative oriC. In the other 25 Par3 strains, the parAB genes are located near each other but farther than 10% of the chromosome size away from the nearest parS site (ParAB-S strains) (Fig. 1). These include all five strains of Helicobacter sp. in the database, in which all predicted parS sites are found within their respective 3% oriC regions but more than 20% of the genome size away from the parAB loci. The conserved positional relationships of parA, parB, parS, and oriC in Par3 strains suggest that these genes and sites have coevolved and that there is selective pressure promoting their proximity.

Phylogenetic analysis of the ParABS system in primary chromosomes.We next examined whether the number and relative chromosomal locations of the different putative ParABS components encoded by the 400 primary chromosomes in the NCBI database correlate with the phylogenetic relationships of these strains (Fig. 2). In most cases, the numbers of parS sites found in different strains of the same species vary by no more than one or two sites (Fig. 2). However, in a few genera, such as Lactobacillus, the number of putative parS sites predicted per strain varied by more than 10.

Our analyses also revealed that different ParABS profiles tend to cluster among phylogenetically related species, genera, and, in many cases, classes and phyla (Fig. 2). Many of the Par0 strains are clustered in two branches of Gammaproteobacteria that include Escherichia, Yersinia, Salmonella, Buchnera, and Haemophilus genera and in the Mesoplasma/Mycoplasma/Ureaplasma/Candidatus branches of the Firmicute class. Moreover, all but 2 of the 37 ParBS strains are in the closely related Staphylococcus, Streptococcus, and Lactococcus genera. Finally, most of ParAB-S strains belong to several closely related species of Bacteroides and Gammaproteobacteria. There are some notable exceptions to the correlation between ParABS profiles and phylogeny (Fig. 2). For example, while Anaplasma phagocytophilum is a Par0 strain, Anaplasma marginale is a Par3 strain. Thus, in some species loss of the Par loci appears to have occurred relatively recently in their evolution.

Secondary chromosomes do not have the parS sequences found on primary chromosomes.ParABS systems encoded by the secondary chromosomes of B. cenocepacia and V. cholerae have been described (13, 60). However, the parS sites present on chromosomes II and III in B. cenocepacia and on V. cholerae chromosome II differ significantly in primary sequence and, in the case of V. cholerae, structure, from parS sites on primary chromosomes (13, 60). Thus, it was not surprising that these previously identified parS2 and parS3 sites were not identified in the search described above. However, it was surprising that of the over 1,000 putative parS1 sites identified and discussed above, none were found on secondary chromosomes, despite the fact that these replicons comprise 4.9% of the total DNA sequence in the NCBI database. This underrepresentation of parS1 sites on secondary chromosomes was not observed when the search was repeated using the scrambled parS matrices; in these searches, 8 of the 112 (7%) predicted sites were found on secondary chromosomes. The complete absence of parS1 sites from secondary chromosomes suggests that there is selective pressure against the occurrence of such sequences on secondary chromosomes.

Identification of putative parS sites on secondary chromosomes.Thirty-seven replicons in the NCBI database are annotated as secondary chromosomes, including five replicons annotated as chromosome III and two annotated as linear chromosomes (Table 1). For our analyses, the 2.1-Mbp pGMI1000MP plasmid of Ralstonia solanacearum was added to this list, since all other Ralstonia spp. strains carry a second chromosome of approximately the same size as pGMI1000MP. Unlike parS sites on primary chromosomes, the characterized parS sites on the secondary chromosomes of B. cenocepacia and V. cholerae vary significantly both in primary sequence and in structure. The V. cholerae parS2 sites are 15-bp sequences composed of 7-nucleotide inverted repeats separated by a central base (60), while the parS sites on B. cenocepacia chromosome II and chromosome III and the predicted Ralstonia solanacearum parS2 site are composed of 14-bp palindromic sequences (13). Although the predicted R. solanacearum parS2 and the B. cenocepacia parS2 and parS3 sequences share similar structures, their primary sequences diverge significantly (13). No putative parS sites on the secondary chromosomes of Brucella spp. have previously been described. However, these replicons are known to encode homologues of the RepABC proteins that mediate segregation of several alphaproteobacterial plasmids (5, 39) and of the secondary (linear) chromosomes of Agrobacterium tumefaciens (29). Like ParB, RepB binds palindromic sites, known as IR sites, that are located in very close proximity to oriC (5, 12, 39). To identify putative parS sites on secondary chromosomes, we constructed five separate matrices using the parS/IR sites from chromosome II of V. cholerae, B. cenocepacia, R. eutropha, and Brucella suis and from chromosome III of B. cenocepacia (see Materials and Methods). Patser was then used to search for sites corresponding to these consensus matrices in all 706 replicons in the NCBI database. The results of these searches are summarized in Table 1 and described below (see also Table S3 in the supplemental material).

Sequence homology, number, and distribution of parS sites on secondary chromosomes.Using the five consensus sequences described above, a total of 151 putative parS sites were identified on 28 of the 38 secondary chromosomes in the NCBI database (Table 1). In contrast with parS1 sites, which are highly conserved among diverse species (Fig. 1A), the sequences of parS/IR sites found on secondary chromosomes were family specific; for example, the search using the V. cholerae parS2 consensus matrix identified parS2 sites on all the second chromosomes of the Vibrionaceae/Photobacteriaceae species, but not on the second chromosomes in species outside of this family. Overall, there is significant divergence of the parS2 sequences found in different families (Fig. 1B). This observation is consistent with the idea that secondary chromosomes have significantly more diverse evolutionary histories than primary chromosomes. Some similarities, however, were observed among sites encoded by secondary replicons within the same or related families (Fig. 1B). For example, all Burkholderia chromosome II and chromosome III and Ralstonia chromosome II parS sites conform to the sequence 5′-TTN(4)CGN(4)AA-3′. Interestingly, no putative parS3 sites were predicted on the third chromosome of B. xenovorans, in contrast to chromosome III of the four other sequenced Burkholderia spp., in which six to seven putative sites/chromosome were identified (Table 1). These findings suggest that the third chromosome of Burkholderia xenovorans may have a distinct evolutionary lineage from that of its counterparts in other Burkholderia strains.

There was significant variability in the number of putative parS sites identified on the secondary chromosomes in different strains (Fig. 3B). All of the secondary chromosomes in Vibrio species contain 6 to 16 putative parS sites, whereas the secondary chromosomes in Brucella and Ralstonia species contain only 2 to 3 predicted parS sites (Table 1). As was observed with parS1 sites, most putative secondary chromosome parS sites are located in close proximity to the oriCs of their respective replicons (Fig. 4B).

A key observation from the searches using the five parS2/parS3 consensus matrices was that, with the exception of one previously described parS2 site on V. cholerae chromosome I (60), no putative parS2 or parS3 sites were found on more than one replicon in a single strain. Thus, even though putative parS2 or parS3 sites were identified on primary chromosomes, they were not detected on primary chromosomes in strains that contained these sites on secondary chromosomes. This finding, coupled with our observation that no strain contains putative parS1 sites on more than one replicon, is consistent with the idea that bacteria require replicon-specific par loci.

All secondary chromosomes that contain putative parS/IR sites also encode putative Par/Rep protein homologues.To identify ParA and ParB homologues on secondary chromosomes, the ParA and ParB proteins encoded by the second chromosomes of V. cholerae (Vc2), B. cenocepacia (Bc2), and B. suis (Bs2, RepA, and RepB) were each compared by BLAST to all proteins in the NCBI database. Since prior studies suggest that Par proteins encoded on secondary chromosomes are more diverse than those encoded on primary chromosomes, we set the minimum BLAST score to 200 for these analyses. Consistent with the observations of Dubarry et al. (13), our findings suggest that Par/Rep proteins encoded on secondary chromosomes, in contrast to ParA and ParB proteins encoded on primary chromosomes, cluster in distinct phylogenetic groups (Table 1) and are more closely related to plasmid-borne partitioning proteins than to those encoded on primary chromosomes. Homologues of both Vc2 ParA and ParB were identified on the secondary chromosomes of all Vibrio strains and on plasmids in 10 strains of Yersinia sp., Salmonella sp., and Shigella sp. but not on any primary chromosomes. Homologues of Bc2 ParA and ParB were identified on chromosome II of all 13 Burkholderia and Ralstonia strains and on 4 of the 5 third chromosomes of Burkholderia sp. strains. Interestingly, the only Burkholderia sp. third chromosome not predicted to encode ParA and ParB homologues was that of Burkholderia xenovorans, the only third chromosome that was not predicted to encode a parS3 site (Table 1). As shown in Table 1, the Bc2 ParA and ParB are much better conserved on Burkholderia second chromosomes than on the third chromosomes of Burkholderia sp. or on the second chromosomes of Ralstonia sp. (Table 1). Homologues of both Bs2 RepA and RepB were found on the secondary chromosomes of all 4 Brucella sp., on the linear chromosomes of both Agrobacterium tumefaciens strains, and on a number of plasmids carried by Alphaproteobacteria strains. No primary chromosomes or other secondary chromosomes were found to encode homologues of either Bs2 RepA or RepB.

Overall we found a perfect correlation between the presence of parAB/repAB genes and cognate parS/IR sites on the secondary chromosomes of Vibrio, Burkholderia, Ralstonia, and Brucella genera. No putative parS sites or Bc2, Vc2, or Bs2 ParB/RepB homologues were identified on the secondary chromosomes of Deinococcus radiodurans, Haloarcula marismortui, Pseudoalteromonas haloplanktis, or Leptospira sp., suggesting that if these replicons encode par loci, they are probably not related to those encoded by secondary chromosomes in Burkholderia, Brucella, Ralstonia, or Vibrio species.

Conclusions.Although the consensus sequence matrix we used to search for parS sites was derived from two gram-positive species, putative parS sites were identified on the primary chromosomes of 69% of strains from all branches of prokaryotes, and these sites exhibited a high degree of sequence conservation (Fig. 1 and 2). We found that, for the most part, strains that do not contain parS sites cluster among relatively few branches of the prokaryotic evolutionary tree. This suggests that parS sites (along with Par proteins) evolved very early in the evolution of prokaryotic chromosomes and that the absence of parS, parA, and/or parB in certain strains likely reflects a loss of one or more of these loci in several ancestral species much later in prokaryotic evolution. The near identity of parS sequences among diverse prokaryotic classes is remarkable; binding sites for many other conserved DNA binding proteins, such as LexA and Fur, do not exhibit such a high level of conservation in structure and/or primary sequence (2, 10, 47, 56, 59).

Currently, knowledge of the function of par loci on primary chromosomes is rudimentary. Several studies have implicated a role for par loci in origin localization and segregation (15, 33, 34, 51), in the separation of sister origins (33), and in the regulation of replication (33, 46, 57). Our finding that the vast majority of parS sites are found in the origin-proximal region of the chromosome strongly suggests that the primary function of par loci pertains to this part of the chromosome and that this function is highly conserved among diverse bacteria. The biological function, if any, of the small minority of putative parS sites found in origin-distal regions of the chromosome remains to be deciphered. We found significant variability among the number of parS sites per chromosome: 47 species contain only one putative parS site, while 48 species contain six or more putative sites. The functional significance of this variability awaits future exploration. Since we found that 25% of all strains are Par0 and that par genes and parS1 sites can be deleted from several bacteria (16, 21, 29, 34, 51, 60), it is likely that, in most cases, par loci encoded on primary chromosomes will not prove to be absolutely required for any essential process, including chromosome partitioning.

Conservation of parS sites among diverse genera was not observed in secondary chromosomes, suggesting that the evolutionary lineages of secondary chromosomes, like those of plasmids, are much more diverse than those of primary chromosomes. Unlike the V. cholerae chromosome I-encoded Par system (16), the par loci encoded on V. cholerae chromosome II and on several plasmids are required for the faithful partitioning of these replicons (61). It is possible that par loci encoded on other secondary chromosomes will also prove essential for the partitioning of these replicons.

The absence of parS1 sites from secondary chromosomes and plasmids and the absence (with one exception) of parS2 sites from primary chromosomes suggest an important difference between prokaryotes and eukaryotes. Eukaryotes can utilize a single mitotic apparatus to mediate the segregation of multiple replicons. Assuming that par loci generally play roles in chromosome segregation, our findings suggest that prokaryotes mediate segregation of multiple replicons by utilizing multiple segregation systems. Experimental support for this idea has come from studies of V. cholerae and B. cenocepacia, where replicon-specific par-mediated segregation systems have been described (13, 60). We speculate that the specificity of parS1 and parS2 sequences to their respective replicons reflects a mechanism to avoid partitioning incompatibility, as has been observed in cells harboring plasmids that contain similar parS sites (7).

Our findings point to some obvious areas for future experimental work. For example, it will be interesting to explore whether ParBS species such as Staphylococcus aureus and Streptococcus pyogenes contain a functional orthologue of ParA or whether the ParB/parS in these species function independently of ParA. Additionally, it should be relatively straightforward to test whether the presence of parS1 sites on secondary chromosomes is indeed deleterious to their segregation. Finally, exploration of segregation by the secondary chromosomes on which no parS sites or Par protein homologues were identified may yield novel information regarding chromosome segregation, especially since our findings support the notion that bacterial species with multiple chromosomes require distinct genes and sites to mediate chromosome segregation.