Sequence analysis of the phs operon in Salmonella typhimurium and the contribution of thiosulfate reduction to anaerobic energy metabolism

Abstract

The phs chromosomal locus of Salmonella typhimurium is essential for the dissimilatory anaerobic reduction of thiosulfate to hydrogen sulfide. Sequence analysis of the phs region revealed a functional operon with three open reading frames, designated phsA, phsB, and phsC, which encode peptides of 82.7, 21.3, and 28.5 kDa, respectively. The predicted products of phsA and phsB exhibited significant homology with the catalytic and electron transfer subunits of several other anaerobic molybdoprotein oxidoreductases, including Escherichia coli dimethyl sulfoxide reductase, nitrate reductase, and formate dehydrogenase. Simultaneous comparison of PhsA to seven homologous molybdoproteins revealed numerous similarities among all eight throughout the entire frame, hence, significant amino acid conservation among molybdoprotein oxidoreductases. Comparison of PhsB to six other homologous sequences revealed four highly conserved iron-sulfur clusters. The predicted phsC product was highly hydrophobic and similar in size to the hydrophobic subunits of the molybdoprotein oxidoreductases containing subunits homologous to phsA and phsB. Thus, phsABC appears to encode thiosulfate reductase. Single-copy phs-lac translational fusions required both anaerobiosis and thiosulfate for full expression, whereas multicopy phs-lac translational fusions responded to either thiosulfate or anaerobiosis, suggesting that oxygen and thiosulfate control of phs involves negative regulation. A possible role for thiosulfate reduction in anaerobic respiration was examined. Thiosulfate did not significantly augment the final densities of anaerobic cultures grown on any of the 18 carbon sources tested. On the other hand, washed stationary-phase cells depleted of ATP were shown to synthesize small amounts of ATP on the addition of formate and thiosulfate, suggesting that thiosulfate reduction plays a unique role in anaerobic energy conservation by S. typhimurium.

The cobalamin (coenzyme B12) biosynthetic genes of Escherichia coli

“…It is noteworthy that the phs operon and the cobalamin-dependent pdu operon are located in this region of the Salmonella chromosome; these functions are both absent from E. coli. The nucleotide sequences of the phs genes (25), however, show no evidence of atypical composition. The E. coli shiA (shikimate permease) gene is located in this region and is not found in S. typhimurium.…”

Genomic reconstruction of a novel, deeply branched sediment archaeal phylum with pathways for acetogenesis and sulfur reduction

“…The phs operon codes for a molybdopterin cofactor that uses cysteine residues to bind and break the di-sulfur bonds (Hinsley and Berks, 2002). Homologs for these cysteine residues and for the iron sulfur clusters responsible for electron transfer have been identified on the phsB gene (Heinzinger et al, 1995). Although this gene has been seen in a few non-thiosulfate-reducing organisms, all BLAST hits to our sequences had functional enzymes (Park et al, 2012).…”

Emergence and Prevalence of Non-H 2 S-Producing Salmonella enterica Serovar Senftenberg Isolates Belonging to Novel Sequence Type 1751 in China

“…To clarify the molecular mechanism underlying the lack of H 2 S production by the 17 S. Senftenberg isolates, the phs operon was amplified and sequenced to detect the presence of genetic mutations. The operon, associated with H 2 S production, contains three genes, phsA, phsB, and phsC (24). All primers were designed based on the S. enterica serovar Typhimurium strain LT2 chromosome (NC_003197.1) using Primer Premier software version 6.0 (Premier Biosoft, Palo Alto, CA, USA).…”