POSITIVE AND NEGATIVE TRANSCRIPTIONAL REGULATION OF THE ESCHERICHIA-COLI GLUCONATE REGULON GENE GNTT BY GNTR AND THE CYCLIC-AMP (CAMP)-CAMPRECEPTOR PROTEIN COMPLEX

The gntT gene of Escherichia coil Is specifically induced by gluconate and repressed via catabolite repression. Thus, gluconate is both an i nducer and a repressor of gntT expression since gluconate is a catabol ite-repressing sugar. In a gntR deletion mutant, the expression of a c hromosomal gntT::lacZ fusion is both high and constitutive, confirming that GntR is the negative regulator of gntT. Indeed, GntR binds to tw o consensus gnt operator sites; one overlaps the -10 region of the gnt T promoter, and the other is centered at +120 with respect to the tran scriptional start site. The binding of GntR to these sites was proven in vitro by gel redardation assays and in vivo by site-directed mutage nesis of the binding sites. Binding of GntR to the operators is elimin ated by gluconate and also by 6-phosphogluconate at a 10-fold-higher c oncentration. Interestingly, when gntR deletion strains are grown in t he presence of gluconate, there is a twofold decrease in gntT expressi on which is independent of catabolite repression and binding of GntR t o the operator sites. This novel response of gntR mutants to the induc er is termed ultrarepression. Transcription of gntT is activated by bi nding of the cyclic AMP (cAMP)-cAMP receptor protein (CRP) complex to a CRP binding site positioned at -71 upstream of the gntT transcriptio n start site.