title: Glucose lowers CRP* levels resulting in repression of the lac operon in cells lacking cAMP.
abstract: 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 beta-galactosidase in the absence of cAMP. We have isolated several mutations in the crp gene that confer a CRP* phenotype. The expression of beta-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 beta-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.
mesh_terms: D000262:Adenylyl Cyclases / Q000502:physiology; D002352:Carrier Proteins; D000242:Cyclic AMP / Q000378:metabolism; D002373:Cyclic AMP Receptor Protein / Q000096:biosynthesis / Q000235:genetics*; D004794:Enzyme Repression; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D005784:Gene Amplification; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects*; D005798:Genes, Bacterial; D005947:Glucose / Q000494:pharmacology*; D007763:Lac Operon / Q000235:genetics*; D010957:Plasmids / Q000235:genetics; D017354:Point Mutation; D011401:Promoter Regions, Genetic / Q000235:genetics; D012329:RNA, Bacterial / Q000096:biosynthesis; D012333:RNA, Messenger / Q000096:biosynthesis; D001616:beta-Galactosidase / Q000096:biosynthesis / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D002373:Cyclic AMP Receptor Protein; D012329:RNA, Bacterial; D012333:RNA, Messenger; D000242:Cyclic AMP; D001616:beta-Galactosidase; D000262:Adenylyl Cyclases; D005947:Glucose