ABSTRACT Using a genetic screen we have identified two chromosomal genes, cusRS ( ylcA ybcZ ), from Escherichia coli K-12 that encode a two-component, signal transduction system that is responsive to copper ions. This regulatory system is required for copper-induced expression of pcoE , a plasmid-borne gene from the E. coli copper resistance operon pco . The closest homologs of CusR and CusS are plasmid-borne two-component systems that are also involved in metal responsive gene regulation: PcoR and PcoS from the pco operon of E. coli ; CopR and CopS from the cop operon, which provides copper resistance to Pseudomonas syringae ; and SilR and SilS from the sil locus, which provides silver ion resistance to Salmonella enterica serovar Typhimurium. The genes cusRS are also required for the copper-dependent expression of at least one chromosomal gene, designated cusC ( ylcB ), which is allelic to the recently identified virulence gene ibeB in E. coli K1. The cus locus may comprise a copper ion efflux system, because the expression of cusC is induced by high concentrations of copper ions. Furthermore, the translation products of cusC and additional downstream genes are homologous to known metal ion antiporters.