An Lrp-type transcriptional regulator controls expression of the Bacillus subtilis chromate transporter.

The Bacillus subtilis strain 168 genome contains the chr3N-chr3C genes encoding the Chr3N/Chr3C protein pair of the chromate ion transporter (CHR) superfamily. Chr3N/Chr3C confers chromate resistance in Escherichia coli only when both proteins are expressed. Upstream of chr3N is the chrS gene encoding ChrS, a protein with homology to the Lrp/AsnC family of transcriptional regulators. When the chrS-chr3N-chr3C gene cluster was transferred to E. coli, a diminished level of chromate resistance was observed, as compared with E. coli transformants bearing only the chromate resistance genes, which displayed full resistance. These data suggested that the chrS gene product acts as negative regulator. RT-PCR assays demonstrated that expression of chrS diminishes transcription of the chromate resistance genes in E. coli, and that this repression was overcome by chromate. Electrophoretic mobility shift assays showed that purified ChrS protein specifically binds to the 5' region of chrS. These results indicate that the chr gene cluster forms an operon regulated negatively by ChrS binding to its own gene's regulatory region, and positively by chromate ions. Sequence analysis revealed similar operons in many Bacillales strains, suggesting some adaptive advantage. This is the first example of a bacterial heavy-metal resistance system controlled by an Lrp-type transcriptional regulator.