Mechanism of catabolite repression in the bgl operon of Escherichia coli: involvement of the anti-terminator BglG, CRP-cAMP and EIIA(Glc) in mediating glucose effect downstream of transcription initiation

Background: Expression of the bgl operon of Escherichia coli, involved in t he regulated uptake and utilization of aromatic beta-glucosides, is extreme ly sensitive to the presence of glucose in the growth medium. We have analy sed the mechanism by which glucose exerts its inhibitory effect on bgl expr ession. Results: Our studies show that initiation of transcription from the bgl pro moter is only marginally sensitive to glucose. Instead, glucose exerts a mo re significant inhibition on the elongation of transcription beyond the rho -independent terminator present within the leader sequence. Transcriptional analyses using plasmids that carry mutations in bglG or within the termina tor, suggest that the target for glucose-mediated repression is the anti-te rminator protein, BglG. Introduction of multiple copies of bglG or the pres ence of mutations that inhibit its phosphorylation by Enzyme IIBgl (BglF), result in loss of glucose repression. Studies using crp, cya and crr strain s show that both CRP-cAMP and the Enzyme IIA(Glc) (EIIA(Glc)) are involved in the regulation. Although transcription initiation is normal in a crp, cy a double mutant, no detectable transcription is seen downstream of the term inator, which is restored by a mutation within the terminator. Transcriptio n past the terminator is also partly restored by the addition of exogenous cAMP to glucose-grown cultures of a crp(+) strain. Glucose repression is lo st in the crr mutant strain. Conclusions: The results summarized above indicate that glucose repression in the bgl operon is mediated at the level of transcription anti-terminatio n, and glucose affects the activity of BglG by altering its phosphorylation by BglF. The CRP-cAMP complex is also involved in this regulation. The res ults using the crr mutant suggest a negative role for EIIA(Glc) in the cata bolite repression of the bgl genes.