Respiratory Selenite Reductase from Bacillus selenitireducens Strain MLS10

Abstract

The putative respiratory selenite [Se(IV)] reductase (Srr) from Bacillus selenitireducens MLS10 has been identified through a polyphasic approach involving genomics, proteomics, and enzymology. Nondenaturing gel assays were used to identify Srr in cell fractions, and the active band was shown to contain a single protein of 80 kDa. The protein was identified through liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a homolog of the catalytic subunit of polysulfide reductase (PsrA). It was found to be encoded as part of an operon that contains six genes that we designated srrE, srrA, srrB, srrC, srrD, and srrF. SrrA is the catalytic subunit (80 kDa), with a twin-arginine translocation (TAT) leader sequence indicative of a periplasmic protein and one putative 4Fe-4S binding site. SrrB is a small subunit (17 kDa) with four putative 4Fe-4S binding sites, SrrC (43 kDa) is an anchoring subunit, and SrrD (24 kDa) is a chaperon protein. Both SrrE (38 kDa) and SrrF (45 kDa) were annotated as rhodanese domain-containing proteins. Phylogenetic analysis revealed that SrrA belonged to the PsrA/PhsA clade but that it did not define a distinct subgroup, based on the putative homologs that were subsequently identified from other known selenite-respiring bacteria (e.g., Desulfurispirillum indicum and Pyrobaculum aerophilum). The enzyme appeared to be specific for Se(IV), showing no activity with selenate, arsenate, or thiosulfate, with a Km of 145 ± 53 μM, a Vmax of 23 ± 2.5 μM min−1, and a kcat of 23 ± 2.68 s−1. These results further our understanding of the mechanisms of selenium biotransformation and its biogeochemical cycle. IMPORTANCE Selenium is an essential element for life, with Se(IV) reduction a key step in its biogeochemical cycle. This report identifies for the first time a dissimilatory Se(IV) reductase, Srr, from a known selenite-respiring bacterium, the haloalkalophilic Bacillus selenitireducens strain MLS10. The work extends the versatility of the complex iron-sulfur molybdoenzyme (CISM) superfamily in electron transfer involving chalcogen substrates with different redox potentials. Further, it underscores the importance of biochemical and enzymological approaches in establishing the functionality of these enzymes.

Methane, arsenic, selenium and the origins of the DMSO reductase family

“…Protein representatives from DMSOR family lineages where the function of the enzyme was previously elucidated by biochemical, genetic, or structural methods were selected to be BLAST queries to construct a comprehensive library of sequences. For the families we analyzed, these included PsrA from W. succinogenes 35 , PhsA from S. enterica serovar Typhimurium 36 , SrrA from B. selenitireducens 37 , ArrA from Chrysiogenes arsenatis 82 and B. selenitireducens 83 , ArxA from A. ehrlichii 19 , TtrA from S. enterica sero- www.nature.com/scientificreports/ var Typhimurium 38 , SrdA from B. selenatarsenatis 39 , the arsenate reductase of P. aerophilum 40 , AioA from A. faecalis 84 and Rhizobium sp. NT-26 85 , FmdB from Methanosarcina barkeri 86 , FmdB and FwdB from Methanothermobacter wolfei 87 FdhG from E. coli 44 , D. gigas 45 , and D. desulfuricans 71 , FdhH from E. coli 46 , NAD-dependent formate dehydrogenases from Moorella thermoacetica 88 and Peptoclostridium acidaminophilum 68 , and the F 420 -dependent formate dehydrogenases of Methanococcus maripaludis 70 and M. vannielli 89 .…”

“…We focused on a number of lineages beyond ArxA/ArrA and AioA. Phylogenetic analyses have consistently found that ArxA/ArrA is closely related to a lineage comprised of the catalytic subunits of the respiratory polysulfide reductase of Wolinella succinogenes (PsrA) 35 , thiosulfate reductase of Salmonella enterica serovar Typhymurium (PhsA) 36 , and selenite reductase of Bacillus selenitireducens (SrrA) 37 , as well as a lineage comprised of the catalytic subunits of the respiratory tetrathionate reductase of S. eneterica serovar Typhymurium (TtrA) 38 , the selenate reductase of B. selenatarsenatis 39 , and the arsenate reductase of the archaeon Pyrobaculum aerophilum 40 , whilst AioA is more closely related to formate dehydrogenase N catalytic subunits (FdhG) 1,[24][25][26] . Rigorous phylogenetic analyses of these DMSOR members and closely related lineages has revealed several novel insights into the evolution of DMSORs.…”

A novel dimethylsulfoxide reductase family of molybdenum enzyme, Idr, is involved in iodate respiration by Pseudomonas sp. SCT

“…For PSCT_04485, the putative [3Fe-4S]-binding motif and the cysteine residue predicted to coordinate the guanine dinucleotide molybdopterin cofactor are highlighted in pink and cyan respectively. The cysteine residue was identified by aligning the IdrA sequence with those of the periplasmic nitrate reductase (NapA) of Desulfovibrio desulfuricans (Dias et al, 1999) and selenite reductase (SrrA) of Bacillus selenitireducens (Wells et al, 2019). For PSCT_04484, the putative twin arginine translocation motif and the predicted [2Fe-2S]-binding motif are highlighted in green and yellow, respectively.…”

Bacilli-Mediated Degradation of Xenobiotic Compounds and Heavy Metals

“…Elemental selenium was deposited both inside and outside of the cells. B. selenitireducens produced enzymes to reduce the oxidized forms of arsenic and selenium to their less toxic reduced forms (Wells et al, 2019 ). B. cereus CM100B and B .…”