Identification of a putative metal binding site in a new family of metalloregulatory proteins

The transcription of the ars operon is negatively controlled by the ArsR repressor and induced by arsenite and antimonite. Using hydroxylamine mutagenesis, four arsR mutants were isolated; three were selected for inability to respond to inducers: C32Y, C32F, and C34Y. Each of the three altered proteins still bound specifically to the ars operator, but inducers were less effective in effecting the release of the altered proteins from the DNA. Each of the mutant arsR genes in trans with a reporter gene controlled by the ars promoter repressed expression of the reporter gene, and addition of inducer did not relieve repression. These results suggest that the altered ArsR proteins are defective in the inducer binding site. The fourth arsR mutation, resulting in a H50Y alteration, produced constitutive expression. His-50 is located within a putative helix-turn-helix region of the ArsR protein. We propose that cysteines Cys-32 and Cys-34 in the ArsR protein comprise part of a metal binding motif found in members of the ArsR family of metalloregulatory proteins.