Mechanism of catabolite repression in the bgl operon of Escherichia coli: involvement of the anti-terminator BglG, CRP-cAMP and EIIAGlc in mediating glucose effect downstream of transcription initiation.
BACKGROUND
Expression of the bgl operon of Escherichia coli, involved in the regulated uptake and utilization of aromatic beta-glucosides, is extremely sensitive to the presence of glucose in the growth medium. We have analysed the mechanism by which glucose exerts its inhibitory effect on bgl expression.


RESULTS
Our studies show that initiation of transcription from the bgl promoter is only marginally sensitive to glucose. Instead, glucose exerts a more 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 terminator, suggest that the target for glucose-mediated repression is the anti-terminator protein, BglG. Introduction of multiple copies of bglG or the presence of mutations that inhibit its phosphorylation by Enzyme IIBgl (BglF), result in loss of glucose repression. Studies using crp, cya and crr strains show that both CRP-cAMP and the Enzyme IIAGlc (EIIAGlc) are involved in the regulation. Although transcription initiation is normal in a crp, cya double mutant, no detectable transcription is seen downstream of the terminator, which is restored by a mutation within the terminator. Transcription past the terminator is also partly restored by the addition of exogenous cAMP to glucose-grown cultures of a crp+ strain. Glucose repression is lost 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-termination, and glucose affects the activity of BglG by altering its phosphorylation by BglF. The CRP-cAMP complex is also involved in this regulation. The results using the crr mutant suggest a negative role for EIIAGlc in the catabolite repression of the bgl genes.
Gulati
A
A
Department of Molecular Reproduction, Development and Genetics, Indian Institute of Science, Bangalore 560 012, India.
Mahadevan
S
S
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology; D002373:Cyclic AMP Receptor Protein / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D005947:Glucose / Q000494:pharmacology; D005960:Glucosides / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics; D009876:Operon; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000235:genetics; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D013728:Terminator Regions, Genetic; D014158:Transcription, Genetic
D001426:Bacterial Proteins; C079638:BglF protein, E coli; D002373:Cyclic AMP Receptor Protein; D029968:Escherichia coli Proteins; D005960:Glucosides; D008565:Membrane Proteins; D016601:RNA-Binding Proteins; C064872:antiterminator proteins, Bacteria; D000242:Cyclic AMP; D011494:Protein Kinases; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System; C031629:phosphoenolpyruvate-glucose phosphotransferase; D005947:Glucose