REGULATORY SYSTEMS MODULATING THE TRANSCRIPTION OF THE PECTINASE GENES OF ERWINIA-CHRYSANTHEMI ARE CONSERVED IN ESCHERICHIA-COLI

To depolymerize plant pectin, the phytopathogenic enterobacterium Erwi nia 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 b y the KdgR repressor and are induced in the presence of a pectin catab olic product, 2-keto-3-deoxygluconate (KDG). Transcription of the pect inase genes is dependent on many environmental conditions. Transcripti onal fusions present on low-copy-number plasmids were used to study th e regulation of the pel genes in a heterologous host, Escherichia coli . Some physiological regulations that take place in Er. chrysanthemi a re 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 catab olism 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 b y temperature or nitrogen starvation, observed in Er. chrysanthemi, wa s not conserved in E. coli, suggesting that the mechanisms responsible for these regulations are specific to Er. chrysanthemi. Analysis of d ifferent E. coli mutants allowed some regulators affecting the transcr iption 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 o f numerous genes, appears to affect peI expression but the effects of E. coli hns mutations are not related to osmoregulation. In contrast, this analysis clearly demonstrates the interchangeability of two regul atory systems of E. coli and Er. chrysanthemi:the global control exert ed by the catabolite activator protein CAP and the specific regulation mediated by the KdgR repressor.