A cytochromebdrepressed by a MarR family regulator confers resistance to metals, nitric oxide, sulfide, and cyanide inChromobacterium violaceum

Abstract

ABSTRACTChromobacterium violaceumis a ubiquitous environmental pathogen. Despite its remarkable adaptability, little is known about the mechanisms of stress resistance in this bacterium. Here, in a screen for iron-susceptible transposon mutants, we identified a cytochromebdthat protectsC. violaceumagainst multiple stresses. The two subunits of this cytochromebd(CioAB) are encoded by thecioRABoperon, which also encodes a GbsR-type MarR family transcription factor (CioR). A ΔcioABmutant strain was sensitive to iron and the iron-requiring antibiotic streptonigrin and showed a decrease in siderophore production. Growth curves and survival assays revealed that the ΔcioABstrain was also sensitive to zinc, hydrogen peroxide, nitric oxide, sulfide, and cyanide. Expression analysis showed that the promoter activity of thecioRABoperon and the transcript levels of thecioABgenes were increased in a ΔcioRmutant. CioR bound the promoter region of theciooperonin vitro, indicating that CioR is a direct repressor of its own operon. Expression of theciooperon increased at high cell density and was dependent on the quorum-sensing regulator CviR. As cyanide is also a signal forcioexpression, and production of endogenous cyanide is known to be a quorum sensing-regulated trait inC. violaceum, we suggest that CioAB is a cyanide-insensitive terminal oxidase that allow respiration under cyanogenic growth conditions. Our findings indicate that the cytochromebdCioAB protectsC. violaceumagainst multiple stress agents that are potentially produced endogenously or during interactions with a host.IMPORTANCEThe terminal oxidases of bacterial respiratory chains rely on heme-copper (heme-copper oxidases) or heme (cytochromebd) to catalyze reduction of molecular oxygen to water.Chromobacterium violaceumis a facultative anaerobic bacterium that uses oxygen and other electron acceptors for respiration under conditions of varying oxygen availability. TheC. violaceumgenome encodes multiple respiratory terminal oxidases, but their role and regulation remain unexplored. Here, we demonstrate that CioAB, the single cytochromebdfromC. violaceum, protects this bacterium against multiple stressors that are inhibitors of heme-copper oxidases, including nitric oxide, sulfide, and cyanide. CioAB also confersC. violaceumresistance to iron, zinc, and hydrogen peroxide. This cytochromebdis encoded by thecioRABoperon, which is under direct repression by the MarR-type regulator CioR. In addition, thecioRABoperon responds to quorum sensing and to cyanide, suggesting a protective mechanism of increasing CioAB in the setting of high endogenous cyanide production.