A widely accepted model for catabolite repression posits that phospho-IIA Glc of the bacterial phosphotransferase system activates adenylyl cyclase (AC) activity. For many years, attempts to observe such regulatory properties of AC in vitro have been unsuccessful. To further study the regulation, AC was produced fused to the transmembrane segments of the serine chemoreceptor Tsr. Cells harboring Tsr-AC and normal AC, expressed from the cya promoter on a low copy number vector, exhibit similar behavior with respect to elevation of cAMP levels resulting from deletion of crp, expressing the catabolite regulatory protein. Membrane-bound Tsr-AC exhibits activity comparable with the native form of AC. Tsr-AC binds IIA Glc specifically, regardless of its phosphorylation state, but not the two general phosphotransferase system proteins, enzyme I and HPr; IIA Glc binding is localized to the C-terminal region of AC. Binding to membranes of either dephospho-or phospho-IIA Glc has no effect on AC activity. However, in the presence of an Escherichia coli extract, P-IIA Glc , but not IIA Glc , stimulates AC activity. Based on these findings of a direct interaction of IIA Glc with AC, but activity regulation only in the presence of E. coli extract, a revised model for AC activity regulation is proposed.In Escherichia coli, cAMP, produced by adenylyl cyclase (AC), 5 is an important regulatory molecule, essential for controlling the expression of numerous operons. The cellular levels of cAMP are regulated mainly via effects on AC activity. It has been firmly established that the phosphoenolpyruvate:sugar phosphotransferase system (PTS) plays an important role in the regulation mechanism. Thus, in wild-type but not in PTS mutant cells, exposure to glucose results in decreased cellular cAMP levels; this decrease accounts for the phenomenon of catabolite repression (1).A popular, but never proven, model for the regulation of AC activity is that the phosphorylated form of IIA Glc of the PTS stimulates AC activity; thus, glucose transport is presumed to lead to dephosphorylation of IIA Glc resulting in a de-activation of AC. It has also been observed that strains of E. coli deficient in the cAMP-binding protein, CRP, produce extraordinarily large amounts of cAMP (2). This CRP-dependent regulation of cAMP levels depends on the presence of IIA Glc (3). It has been proposed that the CRP-cAMP complex promotes expression of a phosphatase that converts P-IIA Glc to dephospho-IIA Glc (4). Consequently, in the absence of CRP, a greater proportion of the pool of IIA Glc is in the phospho-form and the AC is more fully activated.One approach to allow a further understanding of the mechanism by which AC is regulated has involved the use of permeable cells. In this case, exposure of the permeable cells to glucose results in inhibition of AC activity (5). In this system, it was discovered that P i is essential for high activity of AC as well as for the capability of the cells to be inhibited by glucose. Because P i was also shown to stimulate PTS ...
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