External pH: an environmental signal that helps to rationalize pel gene duplication in Erwinia chrysanthemi.

The phytopathogenic bacterium Erwinia chrysanthemi produces five major pectate lyases that are key virulence factors in soft-rot disease development. Using transcriptional fusions, we studied the regulation of pelA, pelD, and pelE gene expression as a function of variation of the external pH. pelA and pelD were expressed when bacteria were grown in an acidic medium while pelE was transcribed only in basic medium. Using phenol red, we observed that, in chicory leaves, pH value of infected tissue varies from acidic to basic. Taken together, these findings are discussed in the context of a model unifying both catalysis and regulation to account for pel gene evolution. In particular, we propose that the three isoenzymes are produced sequentially during the infection process.