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Journal of Bacteriology, May 2009, p. 3367-3374, Vol. 191, No. 10
0021-9193/09/$08.00+0     doi:10.1128/JB.00076-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Redundancy of Enzymes for Formaldehyde Detoxification in Pseudomonas putida ^,

Amalia Roca, Jose J. Rodríguez-Herva, and Juan L. Ramos^*

Department of Environmental Protection, Consejo Superior de Investigaciones Científicas, Estación Experimental del Zaidín, Granada, Spain

Received 21 January 2009/ Accepted 10 March 2009

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Pseudomonas putida KT2440 exhibits redundant formaldehyde dehydrogenases and formate dehydrogenases that contribute to the detoxification of formaldehyde, a highly toxic compound. Physical and transcriptional analyses showed that the open reading frame (ORF) PP0328, encoding one of the formaldehyde dehydrogenases, is self-sufficient, whereas the other functional formaldehyde dehydrogenase gene (ORF PP3970) forms an operon with another gene of unknown function. Two formate dehydrogenase gene clusters (PP0489 to PP0492 and PP2183 to PP2186) were identified, and genes in these clusters were found to form operons. All four transcriptional promoters were mapped by primer extension and revealed the presence of noncanonical promoters expressed at basal level in the exponential growth phase and at a higher level in the stationary phase regardless of the presence of extracellular formaldehyde or formate. These promoters were characterized by a 5'-AG-CCA-C/A-CT-3' conserved region between –7 and –16. To determine the contribution of the different gene products to formaldehyde and formate mineralization, mutants with single and double mutations of formaldehyde dehydrogenases were generated, and the effect of the mutations on formaldehyde catabolism was tested by measuring ¹⁴CO₂ evolution from ¹⁴C-labeled formaldehyde. The results showed that both enzymes contributed to formaldehyde catabolism. A double mutant lacking these two enzymes still evolved CO₂ from formaldehyde, suggesting the presence of one or more still-unidentified formaldehyde dehydrogenases. Mutants with single and double mutations in the clusters for formate dehydrogenases were also generated, and all of them were able to metabolize [¹⁴C]formate to ¹⁴CO₂, suggesting a redundancy of functions that was not limited to only the annotated genes. Single and double mutants deficient in formaldehyde dehydrogenases and formate dehydrogenases exhibited longer lag phases than did the parental strain when confronted with concentrations of formaldehyde close to the MICs. This suggests a role for the detoxification system in tolerance to sublethal concentrations of formaldehyde.

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Gene duplication has long been recognized as a major source of new genes and functions. Until recently, it was assumed that gene duplication served as a means to develop new functions; however, several recent case studies and comparisons of genome content have suggested that most new genes have no new functions (4, 34). Instead, paralogous gene pairs are often subfunctionalized and frequently have overlapping activities that allow both complementation and maintenance of the original functions and an independent evolution of activities. This was recently shown for members of the Old Yellow Enzyme family of flavoproteins, which detoxify nitroaromatic compounds in Pseudomonas putida and other microorganisms (35, 36, 38).

Formaldehyde is a ubiquitous pollutant present in air and water. Its annual production is more than 6 million metric tons, and it is used in the manufacturing of resins, formic acid, polyoxymethylene plastics, 1,4-butanediol, and other compounds (25). Formaldehyde is useful as a disinfectant, and formaldehyde toxicity is well known to derive from its ability to react with functional groups in proteins and nucleic acids, making it an efficient bacteriostatic and bactericidal chemical.

In spite of the toxicity of formaldehyde, some microorganisms can use it as their sole source of carbon (22). In addition, several pathways for the biological detoxification of formaldehyde have been described, among which is the concerted action of a formaldehyde dehydrogenase/formate dehydrogenase system that yields CO₂ (22). Enzymes involved in this detoxification process are often redundant and have been found in yeasts, bacteria, plants, and nematodes (11, 16, 28, 37, 39).

The original annotation of the genome of P. putida KT2440 revealed the presence of three potential formaldehyde dehydrogenase genes (PP0328, PP1939, and PP3970), as well as two clusters of genes potentially able to give rise to two different formate dehydrogenase complexes (PP0489 to PP0492 and PP2183 to PP2186) (Table 1) (21). These complexes are glutathione-independent enzymes (24). In a previous study we exposed P. putida KT2440 to sublethal concentrations of HCOH and analyzed global cell responses using microarrays (29). This analysis revealed that expression of the genes potentially able to encode formaldehyde dehydrogenases and formate dehydrogenases did not vary significantly in the presence of the aldehyde, although a number of functions related to the defense against this toxic chemical were induced, including DNA repair enzymes and chaperones that refold damaged proteins (29). It was then argued that formaldehyde dehydrogenases and formate dehydrogenases could be expressed constitutively to safeguard cells against potential damage caused by the internal production of HCOH, a dead-end product in the metabolism of histidine and methoxylated aromatic compounds such as vanillate (2, 13, 17, 30).

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TABLE 1. Genes potentially able to encode formaldehyde dehydrogenase/formate dehydrogenase

To gain further insights into formaldehyde detoxification in P. putida KT2440, we studied the physical and transcriptional organization of the genes in this system and generated appropriate fusions of the promoter regions of the genes encoding formaldehyde dehydrogenase and formate dehydrogenase to 'lacZ. We found that expression of the detoxification system took place from noncanonical promoters, whose expression increased when cells reached the stationary phase regardless of the presence of formaldehyde and formate in the culture medium. We generated a collection of mutants deficient in formaldehyde dehydrogenases and formate dehydrogenases. Analyses of ¹⁴CO₂ evolution from ¹⁴C-labeled substrates confirmed the redundant nature of the enzymes and revealed the presence of one or more other, unidentified formaldehyde dehydrogenases and formate dehydrogenases.

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Bacterial strains and culture media. The bacterial strains in this study are shown in Table 2. Pseudomonas putida KT2440 and its mutant derivatives were grown on M9 minimal medium with glucose (0.5% [wt/vol]) as the carbon source or LB medium (1). Cultures were incubated at 30°C and shaken on an orbital platform operating at 200 strokes per min. The effects of formaldehyde and formate on the growth of P. putida KT2440 and its isogenic mutants were analyzed spectrophotometrically by tracking the increase in turbidity of the cultures every 60 min. At least four time points were used to estimate doubling times in the exponential phase. When required, the following antibiotics were used at the indicated final concentrations (in micrograms per milliliter): ampicillin, 100; chloramphenicol, 30; kanamycin, 50; tetracycline, 20; and gentamicin, 20.

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TABLE 2. Plasmids and strains used in this study

RNA preparation. Pseudomonas putida KT2440 was grown overnight at 30°C on LB medium. Cells were then diluted 100-fold in fresh medium and incubated with shaking until the culture reached a turbidity of about 0.2 at 660 nm, after which 0.5 mM formaldehyde was added. Mineralization assays showed that this concentration guarantees formaldehyde dehydrogenase and formate dehydrogenase activities. The cultures were incubated for 1.5 to 2 h until they reached a turbidity of around 2, the value at which expression from the target promoters takes place (see Results). Cells (10 ml) were harvested by centrifugation (9,000 x g for 15 min) in disposable plastic tubes precooled in liquid nitrogen and kept at –80°C until use. RNA was extracted using the TRI reagent (catalog no. 9738; Ambion, Austin, TX) with the following modifications. The lysis step was carried out at 60°C, and a final digestion step with RNase-free DNase was added at the end of the process, followed by purification with RNeasy columns (catalog no. 74104; Qiagen, Hilden, Germany). The RNA concentration was determined spectrophotometrically at 260 nm, and RNA integrity was assessed by agarose gel electrophoresis.

RT-PCR assays. Reverse transcriptase PCR (RT-PCR) was done with 1 µg RNA in a final volume of 50 µl using the Titan OneTube RT-PCR system according to the manufacturer's instructions (Roche Laboratories). cDNA was synthesized at 42°C for 60 min, and the PCR cycling conditions were as follows: 94°C for 30 s and 30 cycles at an adequate annealing temperature for 45 s and at 72°C for 45 s. The annealing temperature was calculated for each reaction based on the melting temperatures of the pair of primers used. Positive and negative controls were included in all assays. The primers used to test contiguity in the mRNA are available from us on request.

Primer extension. Primer extension reactions were performed as described by Marqués et al. (20), and the sequences of primers used for extension are reported in the figure legends.

Construction of knockout mutant strains by target insertional mutagenesis. For site-directed mutagenesis, we used pCHESIKmGm (Table 2), a derivative of pCHESIKm that was originally constructed by Llamas et al. (19). To facilitate site-directed mutagenesis, 519 and 579 bp of the central region of PP0489 and PP3970 were amplified by PCR using primers that provide SacI and KpnI sites or EcoRI and SacI sites, respectively. The amplified fragments were ligated to pCHESIKmGm digested with the appropriate restriction enzymes. The resulting plasmids were selected in Escherichia coli, and the correct clones, called pAR0489 and pAR3970, were confirmed by restriction analysis. These plasmids were then transferred to KT2440, as described by Choi et al. (5), and Gm^r recombinant clones were selected. Mutant strains with the correct inactivated target genes were confirmed by Southern blotting using as a probe the originally amplified chromosomal fragment that was previously used for mutant construction. A random clone of each target gene bearing the appropriate insertion was kept for further analysis, and the clones were named P. putida AR0489 and P. putida AR3970 (Table 2).

A double mutant with knockouts in two formaldehyde dehydrogenases, called P. putida AR0328/3970, was constructed by site-directed mutagenesis using P. putida PRCC0328 as a parental strain and plasmid pAR3970 as a source to inactivate PP3970. A mutant with knockouts in PP2184 and PP0489 was generated using P. putida PRCC2184 as the parental strain and pAR0489 as the source of the open reading frame (ORF) PP0489 knockout. Double mutants were confirmed by Southern blotting as described above with appropriate probes based on amplified genes. Furthermore, PCR amplification with primers that annealed adjacent to the target gene were further used to confirm the mutation.

β-Galactosidase assays. We constructed fusions of the promoter of different genes to 'lacZ gene in the low-copy-number pMP220 vector. The corresponding promoter regions were amplified by PCR with primers incorporating restriction sites (a PstI site in the primer designed to meet the 3' end and a BamHI site in the primer designed to meet the 5' end) to create a fusion of the promoters to 'lacZ. Upon amplification, DNA was digested with BamHI and PstI and ligated to BglII-PstI-digested pMP220. The fusion constructs were confirmed by DNA sequencing. The plasmids were electroporated into the wild-type P. putida, and the corresponding transformants were grown overnight on LB medium plus tetracycline (18). The cultures were then diluted 100-fold in the same medium, and three aliquots were made; one was kept unchanged as a control, and to the other two we added 0.5 mM formaldehyde or 10 mM formate. Cultures were incubated at 30°C for about 7 h with shaking, and β-galactosidase activity was then assayed in permeabilized whole cells according to Miller's method (23). Assays were run in triplicate and were repeated at least three times.

Mineralization assays. Overnight cultures of P. putida KT2440 or its isogenic mutants in LB medium were diluted 1:100 in the same medium. Bacterial cultures (20 ml) were placed in airtight sealed 250-ml conical flasks containing 5-ml beakers fixed to the bottom of the flasks. For formaldehyde and formate mineralization, 0.5 µCi and 5 µCi of the compound were added, respectively; the final concentration of the chemical in the medium was 0.5 mM in the case of formaldehyde and 5 mM in the case of formate. ¹⁴CO₂ evolved from the ¹⁴C-labeled substrate was fixed in a 5 M NaOH solution within the internal beaker. Mineralization rates were calculated from evolved ¹⁴CO₂ and the specific activity of the initial ¹⁴C-labeled substrate after incubation for 24 h.

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Physical and transcriptional organization of genes encoding formaldehyde dehydrogenases or formate dehydrogenases in KT2440. BLAST analysis of the genome of P. putida KT2440 against the sequences deposited in different data banks confirmed the initial annotation of the genes and clusters involved in formaldehyde metabolism. The genes are listed in Table 1. The translated products of the ORFs PP0328 and PP3970 were found to be highly similar to known proteins with formaldehyde dehydrogenase activity, and a third ORF (PP1939) was found to yield a short truncated peptide. Subsequently, we compared the PP0328 and PP3970 products and found that they were 32% identical. Comparison of these proteins with the Pseudomonas sp. strain Tanaka formaldehyde dehydrogenase revealed that the PP0328 product was almost 93% identical to this protein, whose activity has been confirmed in vitro (33). The PP0328 gene was annotated as fdhA, whereas the PP3970 gene was called fdhB (Table 1).

The PP0328 gene is transcribed in the same direction as are the PP0326 and PP0327 genes, but the functions of the products of these two genes are unknown. The start codon of ORF PP0328 is 301 bp away from the stop codon of the preceding gene (Fig. 1). To test whether these two genes were part of the same transcriptional unit, we carried out RT-PCR analysis using primers based on the 3' end of a gene (PP0327) and the 5' end of PP0328 located immediately downstream. The absence of an amplification band (Fig. 2) in the RT-PCR was interpreted as evidence that ORF PP0328 was a monocistronic unit.

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FIG. 1. Physical organization of genes encoding enzymes for formaldehyde and formate metabolism. (A) Genes that encode formaldehyde dehydrogenases. The fdhA gene encoding formaldehyde dehydrogenase (PP0328) is within the PP0326-to-PP0329 gene cluster, and the fdhB gene encoding formaldehyde dehydrogenase (PP3970) is within the PP3969-to-PP3972 cluster. (B) Genes in the operons that encode multicomponent formate dehydrogenases with the adjacent gene(s) (PP0489 through PP0492 and PP2183 through PP2186).

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FIG. 2. Operon structure of genes encoding enzymes for formaldehyde metabolism. The products resulting from each RT-PCR were separated on agarose gels, as described in Materials and Methods. The genes to be tested as potentially cotranscribed are shown at the top. Lanes 1, 3, 5, 7, 9, 11, and 13 are negative controls without reverse transcriptase. Lanes 2, 4, 6, 8, 10, 12, and 14 are reactions with reverse transcriptase. The size of the amplified band is given below each band. The rightmost and leftmost lanes in panels A and B, as well as the lane between lanes 2 and 3 in panel A and the lane between lanes 8 and 9 in panel B, are size markers. The sizes of the markers from top to bottom are indicated.

The PP3970 ORF is within a five-gene cluster, all genes of which are transcribed in the same direction (Fig. 1). The distance between the stop codon of the PP3969 ORF and the start codon of PP3970 is 148 bp, whereas the distance between PP3970 and the next ORF is only 14 bp. ORF PP3971 and ORF PP3972 are separated by 32 nucleotides (Fig. 1), whereas 230 bp separate the latter from ORF PP3973 (not shown). To test whether the genes were part of the same transcriptional unit, we carried out RT-PCR analysis as described above but with the appropriate primers. We found that PP3970 was transcribed independently of the 5' gene and was cotranscribed with PP3971 but not with PP3972 (Fig. 2). The function of PP3971 is unknown.

For formate dehydrogenase, two clusters of genes that could give rise to two different formate dehydrogenases were identified in the original annotation of the genome of KT2440 (24). The corresponding genes were designated fmd from formate dehydrogenase. In the original annotation of the KT2440 genome, an opal stop codon at a position corresponding to residue 196 in the ORF product was found. Codon 196 is possibly decoded as a selenocysteine residue in a reaction mediated by an L-seryl-t-RNA selenium transferase (PP0493/PP0494), as is the case in E. coli (2, 40). The cluster made up of PP0489 to PP0492 has been designated fmdA through fmdD (Table 1), whereas the other cluster, consisting of PP2183 through PP2186, has been called fmdEFGH (Table 1). The short distance or overlapping nature of adjacent genes in these two clusters (Fig. 1) suggested that the genes are transcribed as operons, as confirmed by RT-PCR assays (Fig. 2). ORFs PP0489 through PP0492 were transcribed in the same direction as were the upstream and downstream genes. RT-PCR assays showed that PP0489 was not transcribed with PP0488, whereas the last gene (PP0492) was transcribed with the 3'-adjacent ORFs, PP0493 and PP0494, which encode the above-mentioned L-seryl-t-RNA transferase, showing transcriptional coupling of the selenocysteine charging system with its target polypeptide (the PP0489 product) (Fig. 2). Regarding the other formate dehydrogenase cluster, ORF PP2182 was 147 nucleotides away from PP2183 and was not transcriptionally coupled to the formate dehydrogenase gene cluster (not shown). PP2187 was transcribed convergently with respect to PP2186 (Fig. 1).

Early microarray data indicated that the level of expression of the four transcriptional units was independent of the presence of formaldehyde or formate in the culture medium (29). To investigate this further, we first determined the transcription start point of all three operons and PP0328, the monocistronic unit, and then constructed the appropriate fusions to 'lacZ in the low-copy-number pMP220 plasmid.

Primer extension revealed multiple 5' ends in the extension of the different mRNAs, and from a transcriptional point of view, we considered the largest, most intense band as the true transcription start point for each promoter (Fig. 3). The lower bands were considered degradation products. We analyzed the sequences upstream from the proposed +1 position and found that the corresponding sequences were relatively less rich in A's. The alignment of the promoters revealed a potential consensus sequence, 5'-AG-CCA-C/A-CT, from –15 to –7 (in the PP0328 promoter [Fig. 4]) and a nonconserved –35 region (not shown). Expression levels for all four promoters were very low or null when cells were in the exponential growth phase and increased notably when cells attained the stationary phase (Fig. 5 shows PP0328 expression). The level of expression along the growth curve was similarly high regardless of the presence of formaldehyde or formate (Fig. 5; see also Fig. S1 in the supplemental material). Fusion to the promoter of PP3970 yielded the lowest level of expression (20 to 40 Miller units) regardless of the growth phase (not shown). When we compared the maximal levels of β-galactosidase activity, we found that PP2184 and PP0489 expression levels were 3- to 10-fold higher than that of PP0328 (Fig. 6).

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FIG. 3. Transcription initiation point of the four transcriptional units of the genes involved in formaldehyde metabolism. RNA was isolated from cells with a turbidity at 660 nm of about 3. (A) Determination of the start point of ORF PP0328. The primer used for extension was 5'-TTCAGGATGACGCCGTGTTC-3'. Lanes 1 and 7, molecular size ladders; lanes 2 to 5, DNA sequencing ladder; lane 6, extension products. (B, C, and D) Similar to panel A, except that the transcription start points correspond to PP3970, PP0489, and PP2183, respectively. The primers used for mapping were 5'-GCCGAAGATGTCGCCTGCT-3', 5'-CCCCACGTACTTCAGCCAGGG-3', and 5'-CAGTAGGGGCAGTGTTACG-3', respectively. (B) Lanes 1 to 3, size ladders; lane 4, extension products. (C) Lanes 1 and 5, size ladders; lanes 2 and 3, A and C sequences of the promoter; lane 4, extension products. (D) Lane 1, size markers; lanes 2 to 5, DNA sequence of the promoter; lane 6, extension product. nt, nucleotides.

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FIG. 4. Alignment of the promoters of the genes involved in formaldehyde metabolism. The alignment of the promoters covers from +1 to –17. The +1 nucleotide is boxed, and the sequences were aligned using the ALIGNMENT program. A nucleotide is included in the consensus sequence if it appears in at least three of the sequences.

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FIG. 5. Time course expression from the PP0328 promoter along the growth curve. A plasmid bearing a fusion of the promoter region of PP0328 to the 'lacZ gene in pMP220 was transformed in the wild-type P. putida KT2440 strain. Cells were grown in LB medium (squares) supplemented or not with 0.5 mM formaldehyde (circles) or 10 mM formate (triangles). At the indicated times, cell growth was monitored as culture turbidity at 660 nm (top panel) and β-galactosidase activity was determined in permeabilized and intact cells (bottom panel). M.U., Miller units.

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FIG. 6. Exponential- and stationary-phase levels of expression in the parental KT2440 strain and the isogenic rpoS-deficient background. Wild-type P. putida KT2440 cells and the isogenic rpoS mutant strain (27) were transformed with the indicated plasmid and were grown in LB medium. The turbidity of the exponential cultures (E) was around 0.8, whereas stationary cells (S) had been incubated for 7 h and exhibited turbidity in the range of 3.5 to 4.0. β-Galactosidase activity is expressed as the average of at least three independent determinations from three independent cultures. WT, wild type; M.U., Miller units.

The expression pattern shown in Fig. 5 is characteristic of sigma-38-dependent promoters (9, 15), and inspection of the derived consensus sequence and its comparison with the proposed sequence recognized by RpoS (5'-CTAYaCT-3') revealed conservation at positions –13 (C), –10 (A/C), –8 (C), and –7 (C). Subsequently, we measured expression from the promoters using transcriptional fusions to 'lacZ in an RpoS-deficient background. We found that expression levels in this sigma background (24) were 10% to 56% lower, suggesting that although RpoS might contribute to the transcription of these promoters, other sigma factors may also drive expression (Fig. 6). These results are in agreement with other studies that have shown that in vivo certain promoters can be transcribed by RNA polymerase with different sigma factors, i.e., the P. putida Pm promoter that is transcribed with RpoH or RpoS (5, 8, 19), and in E. coli a number of promoters show heterogeneity regarding transcription by RNA polymerase with RpoD and RpoS (14, 23, 31, 32). Details of the sigma factor used by RNA polymerase for transcription from the four promoters analyzed here in relation to formaldehyde/formate detoxification await further in vitro studies.

Mutational analysis of formaldehyde dehydrogenases and formate dehydrogenases. (i) Rates of decrease in ¹⁴CO₂ from H¹⁴COH in fdh/fmd mutants. To determine the potential contribution of each of the formaldehyde dehydrogenases and formate dehydrogenases to formaldehyde detoxification, we requested the PP0328, PP2184, and PP0492 mutants from the Pseudomonas putida Reference Culture Collection: (6) and constructed those that were not available in the collection (PP0489 and PP3970 mutants), as described in Materials and Methods. We thus compiled mutants with knockout mutations at PP0328 and PP3970, encoding formaldehyde dehydrogenases; PP0489 and PP0492 mutants in one of the formate dehydrogenase clusters; and PP2184 in the other formate dehydrogenase cluster. We also constructed mutants with double mutations in formaldehyde dehydrogenases (PP0328/PP3970) and double mutations in the formate dehydrogenase clusters (PP2184/PP0489).

To test the potential contribution of each of the above enzymes to formaldehyde and formate metabolism, we carried out in vivo assays to measure the mineralization of ¹⁴C-labeled substrates. To this end, wild-type cells or isogenic mutant cells were grown on LB medium supplemented with 0.5 mM formaldehyde (0.5 µCi). After 24 h we measured evolved ¹⁴CO₂ and found that mutants deficient in either of the two formaldehyde dehydrogenases evolved 25 to 40% less ¹⁴CO₂ than did the parental strain (Table 3). A double PP0328/PP3970 mutant was also constructed and exhibited significant ¹⁴CO₂ evolution when exposed to [¹⁴C]formaldehyde, suggesting that these high levels of ¹⁴CO₂ from formaldehyde were mediated by one or more nonspecific aldehyde dehydrogenases (10). Other enzymes potentially able to catabolize formaldehyde are the glutathione-dependent formaldehyde dehydrogenases (11, 26, 28). In this regard it is worth noting that up to 36 ORFs of the P. putida KT2440 genome were annotated as potential aldehyde dehydrogenase genes. The presence of nonspecific Pseudomonas putida aldehyde dehydrogenases is in agreement with the relatively broad substrate specificity of this type of enzyme (10).

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TABLE 3. Maximal mineralization of [14C]formaldehyde by the wild type and isogenic P. putida mutantsa

PP0489, PP0492, and PP2184 mutants deficient in formate dehydrogenase were also exposed to [¹⁴C]formaldehyde as described above. These mutants also exhibited reduced levels of ¹⁴CO₂ evolution from [¹⁴C]formaldehyde in comparison to the parental strain (Table 3). This suggests that coupling of the two activities contributes to formaldehyde detoxification.

We also tested mineralization of formate by mutants deficient in formaldehyde dehydrogenases or formate dehydrogenases as described above, except that [¹⁴C]formate (5 µCi) was used in this series of assays. We found that the parental strain and the formaldehyde dehydrogenase mutants evolved similar amounts of ¹⁴CO₂ from [¹⁴C]formate, as expected. However, reduced levels (30% to 60%) were found in mutants deficient in either formate dehydrogenase. Since ¹⁴CO₂ evolution also took place from formate in the PP2184/PP0489 double mutant, at least one more, yet-unidentified formate dehydrogenase appears to operate in this strain.

As mentioned above, PP0489 has an early opal stop codon that could yield a selenocysteine formate dehydrogenase alpha subunit; consequently we expected that inactivation of the gene would have a certain effect on formate metabolism. Our results showed decreased evolution of ¹⁴CO₂ from formate and formaldehyde in this mutant, providing indirect evidence indicating that PP0489 yields an active enzyme, although we cannot exclude the possibility that the formate dehydrogenase alpha subunit made by PP2185 could produce an active formate dehydrogenase with FmdB, FmdC, or FmdD. Functional replacement of catalytic subunits in biphenyl dioxygenases involved in the catabolism of xenobiotic compounds (3, 4, 12), as well as in the replacement of catalytic subunits and multicatalytic proteinase complexes (7), has been described.

(ii) Effect of fdh/fmd mutations on growth. The constitutive nature of the expression of formaldehyde dehydrogenases and formate dehydrogenases suggested that they may be necessary for the initial detoxification of HCOH. If this were the case, growth of mutants deficient in formaldehyde dehydrogenases or formate dehydrogenases may be affected in the presence of sublethal concentrations of formaldehyde or formate. We had previously established MICs of 1.5 mM formaldehyde and 15 mM formate for P. putida KT2440 (29). We tested the effect of sublethal formaldehyde concentrations, i.e., 0.3 mM and 1.2 mM, on the growth of the parental strain and the above series of isogenic mutants. At the lowest concentration, the lag phases of the parental strain and those of all tested mutant strains were similar and in the range of 1.3 h (not shown). Thereafter, the parental and mutant cells grew exponentially with doubling times in the range of 60 to 65 min. In contrast, at a higher concentration of formaldehyde, e.g., 1.2 mM, the lag phase of KT2440 was slightly shorter (about 4 h) than those of most of the mutants (5 to 6 h) (Table 4). Once the lag phase was overcome, cells grew exponentially, although at rates in the range of 56 to 100 min, indicating that once the initial shock was overcome and the cells had adapted, they were able to thrive (Table 4). When wild-type cells or formaldehyde or formate dehydrogenase mutants were exposed to 10 mM formate, no significant changes in the lag phase or growth rate in exponential phase were found.

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TABLE 4. Effect of high formaldehyde concentrations on the lag phase and doubling time in the exponential phasea

We also tested whether deficiency in formaldehyde dehydrogenases or formate dehydrogenases had any effect on killing kinetics in the presence of a high concentration of formaldehyde (10 mM). We found that all mutants died at rates similar to those of the parental strain, i.e., survival was 1 out of 10³ to 10⁴ cells/ml after 10 min (not shown), which suggests that the formaldehyde dehydrogenase/formate dehydrogenase system, although relevant for detoxification of formaldehyde, is not the major player.

In summary, our results show that formaldehyde dehydrogenase and formate dehydrogenase activities are redundant in P. putida and that coupling of formaldehyde dehydrogenase and formate dehydrogenase is critical for the efficient removal of formaldehyde. Genes and operons for formaldehyde degradation are preferentially expressed during the stationary phase, although their transcription seems to be only partially dependent on the RpoS sigma factor. The formaldehyde dehydrogenase/formate dehydrogenase system does not guarantee survival in the presence of high concentrations of exogenously added formaldehyde, in agreement with our earlier proposal that survival of P. putida KT2440 under shock conditions is significantly mediated by chaperones and enzymes involved in DNA repair (29). Instead the formaldehyde dehydrogenase/formate dehydrogenase system may represent a safeguard against the internal production of formaldehyde in the cell catabolism of methoxylated compounds that are abundant in soil, the natural ecosystem of the strain, where cells grow at low rates.

 
This study was initially supported by an intramural project of the CSIC and thereafter by "Proyecto de Excelencia" grant CVI-344 from the Junta de Andalucía and PSYSMO (GEN2006-27750-C5-5-E/SYS).

We thank Estrella Duque for providing mutants, M. M. Fandila and C. Lorente for secretarial assistance, and K. Shashok for improving the use of English in the manuscript.

 
* Corresponding author. Mailing address: Consejo Superior de Investigaciones Científicas-EEZ, C/ Profesor Albareda 1, E-18001 Granada, Spain. Phone: 34-958-181608. Fax: 34-958-135740. E-mail: jlramos{at}eez.csic.es

Published ahead of print on 20 March 2009.

Supplemental material for this article may be found at http://jb.asm.org/.

Present address: Department of Biotechnology, UPM-UNIA, Campus de Montegancedo, 28223 Pozuelo de Alarcón, Madrid, Spain.

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Journal of Bacteriology, May 2009, p. 3367-3374, Vol. 191, No. 10
0021-9193/09/$08.00+0     doi:10.1128/JB.00076-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

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