Regulatory systems modulating the transcription of the pectinase genes of Erwinia chrysanthemi are conserved in Escherichia coli

To depolymerize plant pectin, the phytopathogenic enterobacterium Erwinia chrysanthemi produces five isoenzymes of pectate lyases encoded by the five genes pelA, pelB, pelC, pelD and pelE. In Er. chrysanthemi, all genes involved in pectin degradation are specifically controlled by the KdgR repressor and are induced in the presence of a pectin catabolic product, 2-keto-3-deoxygluconate (KDG). Transcription of the pectinase genes is dependent on many environmental conditions. Transcriptional fusions present on low-copy-number plasmids were used to study the regulation of the pel genes in a heterologous host, Escherichia coli. Some physiological regulations that take place in Er. chrysanthemi are conserved in E. coli. The five pel fusions in E. coli are affected by growth phase, catabolite repression and anaerobic growth conditions and are induced in the presence of galacturonate, a sugar whose catabolism leads to the formation of KDG, the inducer of pel transcription in Er. chrysanthemi. Expression of pelE increased with the osmolarity of the culture medium. In contrast, the regulation of pel expression by temperature or nitrogen starvation, observed in Er. chrysanthemi, was not conserved in E. coli, suggesting that the mechanisms responsible for these regulations are specific to Er. chrysanthemi. Analysis of different E. coli mutants allowed some regulators affecting the transcription of the pel genes to be identified. In E. coli, the growth-phase regulation of the pel genes is not dependent on the RpoS sigma factor and the fnr gene is not involved in the increase of pel expression in oxygen-limited conditions. The gene hns, involved in the regulation of numerous genes, appears to affect pel expression but the effects of E. coli hns mutations are not related to osmoregulation. In contrast, this analysis clearly demonstrates the interchangeability of two regulatory systems of E. coli and Er. chrysanthemi: the global control exerted by the catabolite activator protein CAP and the specific regulation mediated by the KdgR repressor.