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

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 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 transposon mutagenesis. A cytochrome () Tn mutant was generated (Δ), and a detail analysis showed that the following: (1) gene (coding for a GbsR-type regulator) was co-transcribed with the two subunits coding genes of the Cytochrome complex (Cytbd), namely, and , based on RT-PCR analysis, and similar gene arrangements were also found in other family strains; (2) the chromate resistance level was dramatically decreased and chromate reduction efficiency also decreased in strain Δ compared to the wild-type and a complemented strain (Δ-C); (3) Cytbd could catalyze the decomposition of HO according to the analyses of HO decomposition ability, cellular HO contents, HO inhibition zone, and HO 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 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 bacterial one hybrid system and 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 HO to decrease cellular oxidative stress. Thus, the regulation and function of Cytbd may explain why sulfate could enhance chromate reduction.