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Key player in effecting positive control of energy metabolism in bacteria.
cAMP receptor protein, or CRP, by binding to operon control sequences effects an upregulation in gene expression, i.e., by acting as a transcription factor. This DNA binding does not occur, however, unless the protein has been activated through the binding of cyclic AMP (cAMP), which in turn does not occur unless cAMP levels are sufficiently high within the cell.
High cAMP levels do not occur unless the cell is starved for energy. CRP thus functions in the positive control of gene expression particularly when catabolism in bacterial cells is not occurring at sufficiently high levels. This occurs, for example, when the cell is exposed to insufficient glucose concentrations.
When glucose densities are high then cAMP levels within cells are low, CRP is not active, and catabolite repression of operons that are dedicated to the use of alternative energy sources are suppressed (that is, gene expression from those operons is reduced). When glucose densities drop, however, the cell can experience a drop in ATP production. The result is production of cAMP, activation of CRP, and increased expression of these alternative catabolic operons.
cAMP receptor protein was characterized in E. coli with regard to the Lac operon. That is, when glucose is sufficiently abundant in the environment, utilization of also abundant lactose is suppressed. This situation changes, however, when glucose becomes sufficiently depleted, in which case the cell switches over to a much more intense exploitation of lactose. Glucose, that is, provides more "bang for the buck" to the cell than lactose, so lactose is relatively ignored so long as glucose is sufficiently available. CRP is also known as catabolite gene activator protein or CAP.
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