Figure 2.

SirR regulates sulfite reduction via regulation of sirA expression. (a) Sulfite reduction is indicated by loss of sulfite (SO₃) and accumulation of hydrogen sulfide (H₂S). Loss of sirR completely abolished sulfite reduction. Complementation of sirR restored sulfite reduction to wild type levels. (b) Activity of the sirA promoter was measured by lacZ-promoter fusions. Beta-galactosidase activity was measured after overnight aerobic growth or anerobic growth with sulfite as the sole electron acceptor. Both sirR and crp were required for expression of the sulfite reductase operon, as indicated by loss of lacZ expression in the ΔsirR and Δcrp mutants. (c) Expression of sirR and sirA in the wild-type MR-1 and Δcrp mutant. CRP was required for wild-type level expression of sirR and sirA, which indicated that CRP was required for optimal expression of sirR as well as sirA. *p < 0.05, **p < 0.005, ANOVA.