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Sulfate-reducing bacteria (SRB) are sensitive to low concentrations of nitrite, and nitrite has been used to control SRB-related biofouling in oil fields.Desulfovibrio vulgaris Hildenborough, a model SRB, carries a cytochromec -type nitrite reductase (nrfHA ) that confers resistance to low concentrations of nitrite. The regulation of this nitrite reductase has not been directly examined to date. In this study, we show that DVU0621 (NrfR), a sigma54-dependent two-component system response regulator, is the positive regulator for this operon. NrfR activates the expression of thenrfHA operon in response to nitrite stress. We also show thatnrfR is needed for fitness at low cell densities in the presence of nitrite because inactivation ofnrfR affects the rate of nitrite reduction. We also predict and validate the binding sites for NrfR upstream of thenrfHA operon using purified NrfR in gel shift assays. We discuss possible roles for NrfR in regulating nitrate reductase genes in nitrate-utilizingDesulfovibrio spp.IMPORTANCE The NrfA nitrite reductase is prevalent across several bacterial phyla and required for dissimilatory nitrite reduction. However, regulation of thenrfA gene has been studied in only a few nitrate-utilizing bacteria. Here, we show that inD. vulgaris , a bacterium that does not respire nitrate, the expression ofnrfHA is induced by NrfR upon nitrite stress. This is the first report of regulation ofnrfA by a sigma54-dependent two-component system. Our study increases our knowledge of nitrite stress responses and possibly of the regulation of nitrate reduction in SRB.

Regulation of Nitrite Stress Response in Desulfovibrio vulgaris Hildenborough, a Model Sulfate-Reducing Bacterium