In crp mutant strains of Escherichia coli, lacking the catabolite gene activator protein (CAP also known as CRP), cAMP synthesis is substantially increased as compared to wild type strains regardless of the growth medium [Medline]. This increase is also observed in crp mutant strains of Salmonella typhimurium [J Bacteriol].
Increase in cAMP synthesis resulting in an overproduction of cAMP depends mainly on the activation of adenylate cyclase by phosphorylated Enzyme IIAGlc [Microbiology].
Overproduction of cAMP can be visualized on MacConkey agar plate due to cAMP extrusion from E. coli cells [Biochem Biophys Res Commun], a process that was shown to be energy-dependent [J Biol Chem].
It was reported by Krin et al. that the percentage of phosphorylated Enzyme IIAGlc is strongly reduced in glucose-grown wild type strains as compared to glucose-grown crp strains, with no significant difference in the amount of total Enzyme IIAGlc, i.e., phosphorylated plus unphosphorylated [Microbiology]. This finding however finds an explanation in the fact that glucose transport is much slower in crp strains as compared to wild type strains. Indeed, transcription of ptsG (encoding Enzyme IICBGlc) is positively regulated by the CAP-cAMP complex. Therefore, in crp strains, the amount of Enzyme IICBGlc is significantly reduced. Consequently, dephosphorylation of Enzyme IIAGlc by Enzyme IICBGlc is less effective leading to an increased concentration of phosphorylated Enzyme IIAGlc. Subsequently, adenylate cyclase activation by phosphorylated Enzyme IIAGlc is more effective.
In agreement with the proposal that cAMP synthesis in crp strains is mainly dependent on phosphorylated Enzyme IIAGlc, Takahashi et al. reported that Enzyme IIAGlc is largely phosphorylated in crp strains grown in rich medium [Medline]. Interestingly, the same authors also reported that addition of a wild type strain cellular extract to a crp strain cellular extract stimulates dephosphorylation of Enzyme IIAGlc. Therefore it was concluded that, in wild type strains, proteins whose synthesis is dependent on the CAP-cAMP complex may promote dephosphorylation of Enzyme IIAGlc. Unfortunately such proteins were not identified.
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