| Copper-resistant strains of Xanthomonas axonopodis pv. vesicatoria occur on tomato and pepper crops throughout California. The genes conferring copper-resistance to X. axonopodis pv. vesicatoria strain 7882 were sequenced. The genes copLABMG were found to be organized in an operon of about 5 kb. Another gene, copF, coding for a protein related to copper-transporting ATPases, was found downstream of copG. It is not clear if copF is part of the cop operon. The first gene, copL, regulates the expression of the downstream copABMG structural genes. Marker exchange mutagenesis in copL, A, B, M and F resulted in increased copper sensitivity. Although CopL shares 39% identity with an open reading frame (ORF) found upstream of the cop operon in X. arboricola pv. juglandis, marker-exchange mutagenesis in the X. arboricola upstream ORF did not lead to increased copper sensitivity. CopL has 27% identity with an ORF located upstream of copAB in Xylella fastidiosa, and 18.5% identity with an ArsR homologue found in Mesorhizobium loti. The X. axonopodis copper-resistance determinants are most closely related to similar systems found in X. arboricola pv. juglandis ( copAB), Sinorhizobium meliloti (copM), Pseudomonas aeruginosa (copG), and Ralstonia metallidurans (copF). CopA is related to multicopper oxidases, and CopB is similar to a copper-binding outer membrane protein in Pseudomonas syringae pv. tomato. CopM has a CX 2CH heme-binding site found in some cytochromes, and CopG is related to a protein involved in copper resistance in R. metallidurans. The X. axonopodis cop operon shows a unique arrangement of copper resistance genes, not found so far in any other copper-resistant bacteria. We wanted to determine if the cop operon could be induced by other sources of oxidative stress than copper, such as cadmium chloride and tert-butyl hydroperoxide (tBOOH). At least five transcripts were detected after exposure to tBOOH involving the DNA strand antisense to the direction of transcription of copL and overlapping with copL. DNA sequence analysis did not reveal the presence of genes known to be involved in oxidative stress resistance in Xanthomonas . Although we showed that the two systems can work independently, this unique arrangement of overlapping transcripts suggests that copper and oxidative stress resistance could be expressed in a coordinate fashion. |
