Cross-induction of glc and ace operons of Escherichia coli attributable to pathway intersection. Characterization of the glc promoter.

The metabolic pathways specified by the glc and ace operons in Escherichia coli yield glyoxylate as a common intermediate, which is acted on by two malate synthase isoenzymes: one encoded by glcB and the other by aceB. Null mutations in either gene exhibit no phenotype, because of cross-induction of the ace operon by glycolate and the glc operon by acetate. In this study, the regulation of the glc operon, comprising the structural genes glcDEFGB, was analyzed at the molecular level. This operon, activated by growth on glycolate, is transcribed as a single message and is under the positive control of GlcC encoded by a divergent gene. Expression of the glc operon is strongly dependent on the integration host factor (IHF) and is repressed by the global respiratory regulator ArcA-P. In vitro gel-shift experiments demonstrated direct binding of the promoter DNA to IHF and ArcA-P. Mutant analysis indicated that cross-induction of the glc operon by acetate is mediated by the GlcC protein that recognizes the compound as an alternative effector. The similar pattern of regulation of the Glc and Ace systems by IHF and ArcA-P ensures their effective cross-induction.

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