Participation of Regulator AscG of the β-Glucoside Utilization Operon in Regulation of the Propionate Catabolism Operon

Abstract

The asc operon of Escherichia coli is one of the cryptic genetic systems for β- d -galactoside utilization as a carbon source. The ascFB genes for β- d -galactoside transport and catabolism are repressed by the AscG regulator. After genomic SELEX screening, AscG was found to recognize and bind the consensus palindromic sequence TGAAACC-GGTTTCA. AscG binding was detected at two sites upstream of the ascFB promoter and at three sites upstream of the prpBC operon for propionate catabolism. In an ascG -disrupted mutant, transcription of ascFB was enhanced, in agreement with the repressor model of AscG. This repression was indicated to be due to interference of binding of cyclic AMP-CRP to the CRP box, which overlaps with the AscG-binding site 1, as well as binding of RNA polymerase to the promoter. Under conditions of steady-state E. coli growth in a rich medium, the intracellular level of AscG stayed constant at a level supposedly leading to tight repression of the ascFB operon. The level of prpR , encoding the activator of prpBCDE , was also increased in the absence of AscG, indicating the involvement of AscG in repression of prpR . Taken together, these data suggest a metabolic link through interplay between the asc and prp operons.