Glucose lowers CRP* levels resulting in repression of the lac operon in cells lacking cAMP

CRP—cAMP‐dependent operons of Escherichia coli can be expressed in cells lacking functional adenylate cyclase when they carry a second‐site mutation in the crp gene (crp*). It is known that the expression of these operons is repressed by glucose, but the molecular mechanism underlying this cAMP‐independent catabolite repression has been a long‐standing mystery. Here we address the question of how glucose inhibits the expression of β‐galactosidase in the absence of cAMP. We have isolated several mutations in the crp gene that confer a CRP* phenotype. The expression of β‐galactosidase is reduced by glucose in cells carrying these mutations. Using Western blotting and/or SDS—PAGE analysis, we demonstrate that glucose lowers the cellular concentration of CRP* through a reduction in crp* mRNA levels. The level of CRP* protein correlates with β‐galactosidase activity. When the crp promoter is replaced with the bla promoter, the inhibitory effect of glucose on crp* expression is virtually abolished. These data strongly suggest that the lowered level of CRP* caused by glucose mediates catabolite repression in cya− crp* cells and that the autoregulatory circuit of the crp gene is involved in the down‐regulation of CRP* expression by glucose.