Genetic Analysis of Citrobacter sp.86 Reveals Involvement of Corrinoids in Chlordecone and Lindane Biotransformations.

Chlordecone (Kepone®) and γ-hexachlorocyclohexane (γ-HCH or lindane) have been used for decades in the French West Indies (FWI) resulting in long-term soil and water pollution. In a previous work, we have identified a new Citrobacter species (sp.86) that is able to transform chlordecone into numerous products under anaerobic conditions. No homologs to known reductive dehalogenases or other candidate genes were found in the genome sequence of Citrobacter sp.86. However, a complete anaerobic pathway for cobalamin biosynthesis was identified. In this study, we investigated whether cobalamin or intermediates of cobalamin biosynthesis was required for chlordecone microbiological transformation. For this purpose, we constructed a set of four Citrobacter sp.86 mutant strains defective in several genes belonging to the anaerobic cobalamin biosynthesis pathway. We monitored chlordecone and its transformation products (TPs) during long-term incubation in liquid cultures under anaerobic conditions. Chlordecone TPs were detected in the case of cobalamin-producing Citrobacter sp.86 wild-type strain but also in the case of mutants able to produce corrinoids devoid of lower ligand. In contrast, mutants unable to insert the cobalt atom in precorrin-2 did not induce any transformation of chlordecone. In addition, it was found that lindane, previously shown to be anaerobically transformed by Citrobacter freundii without evidence of a mechanism, was also degraded in the presence of the wild-type strain of Citrobacter sp.86. The lindane degradation abilities of the various Citrobacter sp.86 mutant strains paralleled chlordecone transformation. The present study shows the involvement of cobalt-containing corrinoids in the microbial degradation of chlorinated compounds with different chemical structures. Their increased production in contaminated environments could accelerate the decontamination processes.

Transformation of the recalcitrant pesticide chlordecone by Desulfovibrio sp.86 with a switch from ring-opening dechlorination to reductive sulfidation activity.

The insecticide chlordecone has been used in the French West Indies for decades, resulting in long term pollution, human health problems and social crisis. In addition to bacterial consortia and Citrobacter sp.86 previously described to transform chlordecone into three families of transformation products (A: hydrochlordecones, B: polychloroindenes and C: polychloroindenecarboxylic acids), another bacterium Desulfovibrio sp.86, showing the same abilities has been isolated and its genome was sequenced. Ring-opening dechlorination, leading to A, B and C families, was observed as previously described. Changing operating conditions in the presence of chlordecone gave rise to the formation of an unknown sulfur-containing transformation product instead of the aforementioned ones. Its structural elucidation enabled to conclude to a thiol derivative, which corresponds to an undocumented bacterial reductive sulfidation. Microbial experiments pointed out that the chlordecone thiol derivative was observed in anaerobiosis, and required the presence of an electron acceptor containing sulfur or hydrogen sulfide, in a confined atmosphere. It seems that this new reaction is also active on hydrochlordecones, as the 10-monohydrochlordecone A1 was transformed the same way. Moreover, the chlordecone thiol derivative called F1 was detected in several chlordecone contaminated mangrove bed sediments from Martinique Island, highlighting the environmental relevance of these results.

Elucidation of the trigonelline degradation pathway reveals previously undescribed enzymes and metabolites.

Trigonelline (TG; N-methylnicotinate) is a ubiquitous osmolyte. Although it is known that it can be degraded, the enzymes and metabolites have not been described so far. In this work, we challenged the laboratory model soil-borne, gram-negative bacterium Acinetobacter baylyi ADP1 (ADP1) for its ability to grow on TG and we identified a cluster of catabolic, transporter, and regulatory genes. We dissected the pathway to the level of enzymes and metabolites, and proceeded to in vitro reconstruction of the complete pathway by six purified proteins. The four enzymatic steps that lead from TG to methylamine and succinate are described, and the structures of previously undescribed metabolites are provided. Unlike many aromatic compounds that undergo hydroxylation prior to ring cleavage, the first step of TG catabolism proceeds through direct cleavage of the C5-C6 bound, catalyzed by a flavin-dependent, two-component oxygenase, which yields (Z)-2-((N-methylformamido)methylene)-5-hydroxy-butyrolactone (MFMB). MFMB is then oxidized into (E)-2-((N-methylformamido) methylene) succinate (MFMS), which is split up by a hydrolase into carbon dioxide, methylamine, formic acid, and succinate semialdehyde (SSA). SSA eventually fuels up the TCA by means of an SSA dehydrogenase, assisted by a Conserved Hypothetical Protein. The cluster is conserved across marine, soil, and plant-associated bacteria. This emphasizes the role of TG as a ubiquitous nutrient for which an efficient microbial catabolic toolbox is available.

Draft Genome Sequence of Propane- and Butane-Oxidizing Actinobacterium Rhodococcus ruber IEGM 231.

We report a draft genome sequence of Rhodococcus ruber IEGM 231, isolated from a water spring near an oil-extracting enterprise (Perm region, Russian Federation). This sequence provides important insights into the genetic mechanisms of propane and n-butane metabolism, organic sulfide and beta-sitosterol biotransformation, glycolipid biosurfactant production, and heavy metal resistance in actinobacteria.

Microbial urate catabolism: characterization of HpyO, a non-homologous isofunctional isoform of the flavoprotein urate hydroxylase HpxO.

In aerobic cells, urate is oxidized to 5-hydroxyisourate by two distinct enzymes: a coenzyme-independent urate oxidase (EC 1.7.3.3) found in eukaryotes and bacteria like Bacillus subtilis and a prokaryotic flavoprotein urate hydroxylase (HpxO) originally found in some Klebsiella species. More cases of analogous or non-homologous isofunctional enzymes (NISE) for urate catabolism have been hypothesized by inspecting bacterial genomes. Here, we used a functional complementation approach in which a candidate gene for urate oxidation is integrated by homologous recombination in the Acinetobacter baylyi ADP1 genome at the locus of its original hpxO gene. Catabolism of urate was restored in A. baylyi ADP1 expressing a FAD-dependent protein from Xanthomonas campestris, representing a new urate hydroxylase family that we called HpyO. This enzyme was kinetically characterized and compared with other HpxO enzymes. In contrast to the latter, HpyO is a typical Michaelian enzyme. This work provides the first experimental evidences for the function of HpyO in bacterial urate catabolism and establishes it as a NISE of HpxO.

Genome duplication in the teleost fish Tetraodon nigroviridis reveals the early vertebrate proto-karyotype.

Tetraodon nigroviridis is a freshwater puffer fish with the smallest known vertebrate genome. Here, we report a draft genome sequence with long-range linkage and substantial anchoring to the 21 Tetraodon chromosomes. Genome analysis provides a greatly improved fish gene catalogue, including identifying key genes previously thought to be absent in fish. Comparison with other vertebrates and a urochordate indicates that fish proteins have diverged markedly faster than their mammalian homologues. Comparison with the human genome suggests approximately 900 previously unannotated human genes. Analysis of the Tetraodon and human genomes shows that whole-genome duplication occurred in the teleost fish lineage, subsequent to its divergence from mammals. The analysis also makes it possible to infer the basic structure of the ancestral bony vertebrate genome, which was composed of 12 chromosomes, and to reconstruct much of the evolutionary history of ancient and recent chromosome rearrangements leading to the modern human karyotype.

Global heterochromatic colocalization of transposable elements with minisatellites in the compact genome of the pufferfish Tetraodon nigroviridis.

Because of its unusual high degree of compaction and paucity of repetitive sequences, the genome of the smooth pufferfish Tetraodon nigroviridis is the subject of a well-advanced sequencing project. An astonishing diversity of transposable elements not found in the human and the mouse has been observed in the genome of T. nigroviridis. Due to the difficulty of assembling repeat-rich regions, the whole genome shotgun sequencing approach will probably fail to reveal the general organisation of this compact vertebrate genome. Therefore, in order to gain new insights into the global distribution pattern of repeated DNA in the genome of T. nigroviridis, we have reconstructed partial/complete repetitive sequences from data generated by the genome project and performed double-colour fluorescent in situ hybridization (FISH) analysis for representatives of three major categories of repeated sequences including two minisatellites (ms100 and ms104), two DNA transposons (Tol2 and Buffy1) and two non-long terminal repeat (LTR) retrotransposons (Rex3 and Babar). We show that DNA transposons and retroelements very frequently colocalize with minisatellites and mostly accumulate within heterochromatic regions. These results, which have not been reported so far for the fugu Takifugu rubripes, show that repeated elements are generally excluded from gene-rich regions in T. nigroviridis and underline the extreme degree of compartmentalization of this compact genome. The genome organization of the pufferfish is clearly different from that observed in humans, where repeated sequences make up an important fraction of euchromatic DNA, and is more similar to that observed in the fruit fly Drosophila melanogaster.

An active non-LTR retrotransposon with tandem structure in the compact genome of the pufferfish Tetraodon nigroviridis.

The fish retrotransposable element Zebulon encodes a reverse transcriptase and a carboxy-terminal restriction enzyme-like endonuclease, and is related phylogenetically to site-specific non-LTR retrotransposons from nematodes. Zebulon was detected in the pufferfishes Tetraodon nigroviridis and Takifugu rubripes, as well as in the zebrafish Danio rerio. Structural analysis suggested that Zebulon, in contrast to most non-LTR retrotransposons, might be able to retrotranspose as a partial tandem array. Zebulon was active relatively recently in the compact genome of T. nigroviridis, in which it contributed to the extension of intergenic and intronic sequences, and possibly to the formation of genomic rearrangements. Accumulation of Zebulon together with other retrotransposons was observed in some heterochromatic chromosomal regions of the genome of T. nigroviridis that might serve as reservoirs for active elements. Hence, pufferfish compact genomes are not evolutionarily inert and contain active retrotransposons, suggesting the presence of mechanisms allowing accumulation of retrotransposable elements in heterochromatin, but minimizing their impact on euchromatic regions. Homologous recombination between partial tandem sequences eliminating active copies of Zebulon and reducing the size of insertions in intronic and intragenic regions might represent such a mechanism.

Analysis of 148 kb of genomic DNA of Tetraodon nigroviridis covering an amylase gene family.

We have sequenced and analysed a 148 kb genomic region of Tetraodon nigroviridis, a teleost fish with a compact genome. Several genes were identified by comparison with genomic or transcript sequences of other species, informatic prediction and screening of a cDNA library. As expected for a compact genome, sizes of the identified genes and introns are very small, and intergenic distances are short. Among identified genes, three code for amylases. As in mammals, these genes are linked, but they are found in a small region of less than 11 kb. These results represent the first description of a genomic sequence larger than 100 kb in this species. Synteny with the human genome is restricted to three regions corresponding to human 1p32.3, 1p13.3 and 1p21.1.

Conservation of the T-cell receptor alpha/delta linkage in the teleost fish Tetraodon nigroviridis.

T-cell specific receptors (TCR) are present in all groups] from the jawed vertebrates to the mammals. In teleosts, however, the genes encoding the gamma- and delta-chains have not yet been found, the alpha- and beta-chains have been characterized mainly at the expression level, and genomic organization of these loci remains largely unknown. Here we describe both the genomic organization of the TCR alpha/delta locus in Tetraodon nigroviridis and the transcription of TCRA and TCRD. The TCR alpha/delta locus consists of 13 V alpha/delta segments, a Calpha gene, and 12 Jalpha segments, followed by a Cdelta gene, two Jdelta segments, and several Ddelta segments. However, the genomic organization found in this teleost differs significantly from that which has been observed in mammals and birds: a common set of V segments is used to generate either an alpha- or a delta-chain by genomic inversion, and the size of the locus is small in this vertebrate.