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Sulfate-reducing bacteria are a group of microorganisms that use sulfate as an electron acceptor. These bacteria are useful in the bioremediation of heavy metal pollution since they can reduce/precipitate metals. Previously, we identified the  Alishewanella  strain WH16-1 from soil of a copper and iron mine and determined that it can reduce sulfate and chromate and that it was tolerant to many heavy metals. In this study, we investigated the chromate reduction mechanism of strain WH16-1 through Tn 5  transposon mutagenesis. A cytochrome  bd  ( cytbd ) Tn 5  mutant was generated (Δ cytbd ), and a detail analysis showed that the following: (1) gene  cydE  (coding for a GbsR-type regulator) was co-transcribed with the two subunits coding genes of the Cytochrome  bd  complex (Cytbd), namely,  cydA  and  cydB , based on RT-PCR analysis, and similar gene arrangements were also found in other  Alteromonadaceae  family strains; (2) the chromate resistance level was dramatically decreased and chromate reduction efficiency also decreased in strain Δ cytbd  compared to the wild-type and a complemented strain (Δ cytbd -C); (3) Cytbd could catalyze the decomposition of H 2 O 2  according to the analyses of H 2 O 2  decomposition ability, cellular H 2 O 2  contents, H 2 O 2  inhibition zone, and H 2 O 2  sensitivity tests; (4) surprisingly, chromate was not an inducer of the expression of Cytbd, but sulfate induced expression of Cytbd, and sulfate/sulfide resistance levels were also decreased in the Δ cytbd  strain; (5) the addition of sulfate enhanced the chromate resistance level and reduction efficiency; (6) Cytbd expression was repressed by CydE and derepressed by sulfate based on an  in vivo  bacterial one hybrid system and  in vitro  EMSA tests; and (7) DNA footprinting and short-fragment EMSA tests revealed two binding sites of CydE in its promoter region. All these results showed that Cytbd is negatively regulated by CydE and derepressed by sulfate. In addition, Cytbd contributes to the resistance of sulfate and sulfide, and sulfide could be used as a reductant to reduce chromate. Moreover, Cytbd is essential to decompose H 2 O 2  to decrease cellular oxidative stress. Thus, the regulation and function of Cytbd may explain why sulfate could enhance chromate reduction.

frontiers in microbiology

The Cytochrome bd Complex Is Essential for Chromate and Sulfide Resistance and Is Regulated by a GbsR-Type Regulator, CydE, in Alishewanella Sp. WH16-1