Specific induction of the copper resistance operon (cop) promoter from Pseudomonas syringae was measured by beta-galactosidase production from a cop promoter-lacZ fusion. Induction of the cop promoter in P. syringae pv. syringae required trans-acting factors from copper resistance plasmid pPT23D, from which cop was originally cloned. Tn5 mutagenesis of pPT23D was used to localize two complementation groups immediately downstream from copABCD. Cloning and sequencing of the DNA in this region revealed two genes, copR and copS, expressed in the same orientation as the cop operon but from a separate constitutive promoter. The amino acid sequence deduced from these genes showed distinct similarities to known two-component regulatory systems, including PhoB-PhoR and OmpR-EnvZ. In addition, CopR showed strong similarity to copper resistance activator protein PcoR from Escherichia coli. Functional chromosomal homologs to copRS activated the cop promoter, in a copper-inducible manner, in copper-resistant or -sensitive strains of P. syringae pv. tomato and other Pseudomonas species. This implies that copper-inducible gene regulation is associated with a common chromosomally encoded function, as well as plasmid-borne copper resistance, in Pseudomonas spp.