TRANSCRIPTIONAL REGULATION OF THE ESCHERICHIA-COLI RHAT GENE

Transcriptional regulation of the rhaT gene, one of the operons formin g the rhamnose regulon in Escherichia coli, was studied by fusing its complete or deleted promoter to the reporter gene lacZ, Analysis of be ta-galactosidase activities induced in these constructions grown under different conditions predicted the presence of two putative control e lements: one for the RhaS regulatory protein and activating the gene n ot only by L-rhamnose but also by L-lyxose or L-mannose, the other for cAMP-catabolite repression protein and activating this gene in the ab sence of glucose. Anaerobiosis increased the promoter function two- to threefold with respect to the aerobic condition, Experiments involvin g complementation of strains containing the rhaT-promoter fusion and c arrying a deletion in the rhaS and/or rhaR genes with plasmids bearing the rhamnose regulatory genes showed that rhaT is controlled by a reg ulatory cascade, in which RhaR induces rhaSR and the accumulated RhaS directly activates rhaT.