Role of morphological growth state and gene expression in Desulfovibrio africanus strain Walvis Bay mercury methylation.

The biogeochemical transformations of mercury are a complex process, with the production of methylmercury, a potent human neurotoxin, repeatedly demonstrated in sulfate- and Fe(III)-reducing as well as methanogenic bacteria. However, little is known regarding the morphology, genes, or proteins involved in methylmercury generation. Desulfovibrio africanus strain Walvis Bay is a Hg-methylating δ-proteobacterium with a sequenced genome and has unusual pleomorphic forms. In this study, a relationship between the pleomorphism and Hg methylation was investigated. Proportional increases in the sigmoidal (regular) cell form corresponded with increased net MeHg production but decreased when the pinched cocci (persister) form became the major morphotype. D. africanus microarrays indicated that the ferrous iron transport genes (feoAB), as well as ribosomal genes and several genes whose products are predicted to have metal binding domains (CxxC), were up-regulated during exposure to Hg in the exponential phase. Whereas no specific methylation pathways were identified, the finding that Hg may interfere with iron transport and the correlation of growth-phase-dependent morphology with MeHg production are notable. The identification of these relationships between differential gene expression, morphology, and the growth-phase dependence of Hg transformations suggests that actively growing cells are primarily responsible for methylation, and so areas with ample carbon and electron-acceptor concentrations may also generate a higher proportion of methylmercury than more oligotrophic environments. The observation of increased iron transporter expression also suggests that Hg methylation may interfere with iron biogeochemical cycles.

Genome sequence of the mercury-methylating and pleomorphic Desulfovibrio africanus Strain Walvis Bay.

Desulfovibrio africanus strain Walvis Bay is an anaerobic sulfate-reducing bacterium capable of producing methylmercury (MeHg), a potent human neurotoxin. The mechanism of methylation by this and other organisms is unknown. We present the 4.2-Mb genome sequence to provide further insight into microbial mercury methylation and sulfate-reducing bacteria.

Sequencing intractable DNA to close microbial genomes.

Advancement in high throughput DNA sequencing technologies has supported a rapid proliferation of microbial genome sequencing projects, providing the genetic blueprint for in-depth studies. Oftentimes, difficult to sequence regions in microbial genomes are ruled "intractable" resulting in a growing number of genomes with sequence gaps deposited in databases. A procedure was developed to sequence such problematic regions in the "non-contiguous finished" Desulfovibrio desulfuricans ND132 genome (6 intractable gaps) and the Desulfovibrio africanus genome (1 intractable gap). The polynucleotides surrounding each gap formed GC rich secondary structures making the regions refractory to amplification and sequencing. Strand-displacing DNA polymerases used in concert with a novel ramped PCR extension cycle supported amplification and closure of all gap regions in both genomes. The developed procedures support accurate gene annotation, and provide a step-wise method that reduces the effort required for genome finishing.

Desulfovibrio africanus subsp. uniflagellum subsp. nov., a sulfate-reducing bacterium from a uranium-contaminated subsurface aquifer.

The bacterial strain SR-1(T) was isolated from subsurface sediments of a uranium-contaminated site in Shiprock, New Mexico, USA. Cells are vibrioid and motile by means of a single polar flagellum. Strain SR-1(T) grows on sulfate, oxidizing formate, lactate and H2, but not malate, and ferments pyruvate. The DNA sequences of the 16S rRNA gene and the 16S-23S internal transcribed spacer of strain SR-1(T) showed 99.9 and 99.4 % similarity, respectively, to those of the type strain Desulfovibrio africanus DSM 2603(T). The DNA sequence of the ITS region is 300 bases in length and contains two tRNA genes (tRNA(Ile), tRNA(Ala)). The partial DNA sequence of the dsrAB gene showed 94.6 % amino acid sequence similarity to that of D. africanus. The DNA G+C content of strain SR-1(T) was 62.4 mol% and it showed 72 % DNA-DNA similarity to D. africanus. DNA typing methods that target gene clusters and whole genomes revealed characteristic genomic fingerprints for strain SR-1(T). A small plasmid was detected by gel electrophoresis. On the basis of distinct phenotypic and genotypic characteristics, strain SR-1(T) represents a novel subspecies of D. africanus, for which the name Desulfovibrio africanus subsp. uniflagellum subsp. nov. is proposed. The type strain is SR-1(T) (=JCM 15510(T) =LS KCTC 5649(T)).

Characterization of pNC1, a small and mobilizable plasmid for use in genetic manipulation of Desulfovibrio africanus.

To develop a vector system that facilitates genetic manipulation in Desulfovibrio species, we screened native sulfate-reducing bacteria for small plasmids. A self-replicating plasmid was discovered in Desulfovibrio africanus SR-1. Sequence analysis of this 8568-bp plasmid (pNC1) revealed a G+C content of 47.2% and nine open reading frames. This plasmid has a copy number of six. Compatible hosts include D. africanus and Pseudomonas aeruginosa PA14. Genetic characterization of pNC1 revealed that 53.6% of the plasmid contains genes associated with replication, mobilization, and partitioning. The 1123-bp replicon is composed of a rep gene and four 22-bp iterons. The mobilization operon is composed of three genes with a putative 144-bp oriT. The partitioning operon is composed of parA and parB with a downstream parS. We report the construction of a small pNC1-based cloning vector which transforms D. africanus at high frequencies (approximately 1.5 x 10(3) CFU/microg DNA), is mobilizable at high transfer frequency (4.8 x 10(-4) transconjugants/donor), and is stably maintained under non-selective pressure. This study provides a potential host-vector system for Desulfovibrio gene functional analyses.

Disulfide bond-dependent mechanism of protection against oxidative stress in pyruvate-ferredoxin oxidoreductase of anaerobic Desulfovibrio bacteria.

Oxidative decarboxylation of pyruvate forming acetyl-coenzyme A is a crucial step in many metabolic pathways. In most anaerobes, this reaction is carried out by pyruvate-ferredoxin oxidoreductase (PFOR), an enzyme normally oxygen sensitive except in Desulfovibrio africanus (Da), where it shows an abnormally high oxygen stability. Using site-directed mutagenesis, we have specified a disulfide bond-dependent protective mechanism against oxidative conditions in Da PFOR. Our data demonstrated that the two cysteine residues forming the only disulfide bond in the as-isolated PFOR are crucial for the stability of the enzyme in oxidative conditions. A methionine residue located in the environment of the proximal [4Fe-4S] cluster was also found to be essential for this protective mechanism. In vivo analysis demonstrated unambiguously that PFOR in Da cells as well as two other Desulfovibrio species was efficiently protected against oxidative stress. Importantly, a less active but stable Da PFOR in oxidized cells rapidly reactivated when returned to anaerobic medium. Our work demonstrates the existence of an elegant disulfide bond-dependent reversible mechanism, found in the Desulfovibrio species to protect one of the key enzymes implicated in the central metabolism of these strict anaerobes. This new mechanism could be considered as an adaptation strategy used by sulfate-reducing bacteria to cope with temporary oxidative conditions and to maintain an active dormancy.

Multiple orientations in a physiological complex: the pyruvate-ferredoxin oxidoreductase-ferredoxin system.

Ferredoxin I from Desulfovibrio africanus (Da FdI) is a small acidic [4Fe-4S] cluster protein that exchanges electrons with pyruvate-ferredoxin oxidoreductase (PFOR), a key enzyme in the energy metabolism of anaerobes. The thermodynamic properties and the electron transfer between PFOR and either native or mutated FdI have been investigated by microcalorimetry and steady-state kinetics, respectively. The association constant of the PFOR-FdI complex is 3.85 x 10(5) M(-1), and the binding affinity has been found to be highly sensitive to ionic strength, suggesting the involvement of electrostatic forces in formation of the complex. Surprisingly, the punctual or combined neutralizations of carboxylate residues surrounding the [4Fe-4S] cluster slightly affect the PFOR-FdI interaction. Furthermore, hydrophobic residues around the cluster do not seem to be crucial for the PFOR-FdI system activity; however, some of them play an important role in the stability of the FeS cluster. NMR restrained docking associated with site-directed mutagenesis studies suggested the presence of various interacting sites on Da FdI. The modification of additional acidic residues at the interacting interface, generating a FdI pentamutant, evidenced at least two distinct FdI binding sites facing the distal [4Fe-4S] cluster of the PFOR. We also used a set of various small acidic partners to investigate the specificity of PFOR toward redox partners. The remarkable flexibility of the PFOR-FdI system supports the idea that the specificity of the physiological complex has probably been "sacrificed" to improve the turnover rate and thus the efficiency of bacterial electron transfer.