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

The phytopathogenic bacterium Erwinia chrysanthemi produces five major pect ate lyases that are key virulence factors in soft-rot disease development. Using transcriptional fusions, we studied the regulation of pelA, pelD, and pelf gene expression as a function of variation of the external pH pelA an d pelD were expressed when bacteria were grown in an acidic medium while pe lf was transcribed only in basic medium. Using phenol red, we observed that , in chicory leaves, pH value of infected tissue varies from acidic to basi c. Taken together, these findings are discussed in the context of a model u nifying 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.