New genes involved in chromate resistance in Ralstonia metallidurans strain CH34.

Chromate resistance in Ralstonia metallidurans CH34 is based on chromate efflux catalyzed by ChrA efflux pumps. The bacterium harbors two chromate resistance determinants, the previously known chr(1) on plasmid pMOL28 (genes chrI, chrB(1), chrA(1), chrC, chrE, chrF(1)) and chr(2) on the chromosome (genes chrB(2), chrA(2), chrF(2)). Deletion of the genes chrI, chrC, chrA(2), chrB(2) and chrF(2) influenced chromate resistance and transcription from a chrBp(1) ::lacZ fusion. Deletion of the plasmid-encoded gene chrB(1) did not change chromate resistance or chrBp(1) regulation. Northern hybridization and primer-extension experiments were used to study transcription of the plasmid-encoded chr(1) determinant. Transcription of chrB(1), chrA(1) and chrC was induced by chromate. The presence of sulfate influenced transcription positively. The chrBp(1), chrAp(1) and chrCppromoters showed some similarity to heat-shock promoters. Transcription of the gene rpoH encoding a putative heat-shock sigma factor was also induced by chromate, but rpoH was not essential for chromate resistance. The ChrC protein was purified as a homotetramer and exerted superoxide dismutase activity. Thus, possible regulators for chromate resistance (ChrI, ChrB(1), ChrB(2), ChrF(1), and ChrF(2)) and an additional detoxification system (ChrC) were newly identified as parts of chromate resistance in R. metallidurans.