Cobalt-dependent transcriptional switching by a dual-effector MerR-like protein regulates a cobalt-exporting variant CPx-type ATPase

CoaR associates with and confers cobalt-dependent activation of the coaT operator-promoter. A CoaR mutant (Ser-Asn-Ser) in a carboxyl-terminal Cys-His-Cys motif bound the coaT operator-promoter but did not activate expression in response to cobalt, implicating thiolate and/or imidazole ligands at these residues in an allosteric cobalt binding site. Deletion of 1 or 2 nucleotides from between near consensus, but with aberrant (20 base pairs) spacing, -10 and -35 elements enhanced expression from the coaT operator-promoter but abolished activation by cobalt-CoaR, It is inferred that cobalt effects a transition in CoaR that underwinds the coaT operator-promoter to realign promoter elements, In the absence of cobalt, CoaR represses expression (similar to 50%). CoaR is a fusion of ancestral MerR (mercury-responsive transcriptional activator)- and precorrin isomerase (enzyme of vitamin B-12 biosynthesis)-related sequences. Expression from the coaT operator-promoter was enhanced in a partial mutant of cbiE (encoding an enzyme preceding precorrin isomerase in B-12 biosynthesis), revealing that this pathway "inhibits" coaT expression. Disruption of coaT reduced cobalt tolerance and increased cytoplasmic Co-57 accumulation. coaT-mediated restoration of cobalt tolerance has been used as a selectable marker.