CHARACTERIZATION OF CHROMOSOMAL HOMOLOGS OF THE PLASMID-BORNE COPPER RESISTANCE OPERON OF PSEUDOMONAS-SYRINGAE

Copper-resistant and copper-sensitive strains of Pseudomonas syringae, as well as many other pseudomonads, contain chromosomal DNA homologou s to the plasmid-borne copper resistance operon (copABCD). cop homolog s were cloned from the chromosome of P. syringae pv. tomato PT12.2, wh ich had an elevated level of resistance to copper compared with typica l copper-sensitive strains of other P. syringae pathovars and showed a n unusually high frequency of spontaneous mutation to high levels of c opper resistance. Two chromosomal cop homolog regions were cloned. Hom olog 1 hybridized with copA and copB, and homolog 2 hybridized with co pA, copB, copC, and the copper-responsive regulatory genes copRS. Homo log 1 had no detectable function when transferred to a copper-sensitiv e strain of P. syringae. However, homolog 2 conferred the low level of copper resistance observed with PT12.2 and produced proteins related to CopA and CopC. In addition, homolog 2 conferred a high frequency of mutation to full copper resistance. In a spontaneously mutated deriva tive of the cloned homolog 2 (pCOPH2R) that conferred copper resistanc e, an increased level of CopA was observed. pCOPH2R also supported a h igher level of transcriptional activity of the cop promoter that was f used to lacZ and provided in trans (pCOP38), suggesting that the spont aneous mutation was regulatory, probably involving the copRS homologs. Homolog 2 was similar but not identical to the plasmid-borne cop oper on, and it did not complement site-specific mutations in cop genes.