Catabolite Repression
 
The ability of glucose to inhibit the synthesis of certain enzymes, referred to as the glucose effect, was recognized in bacteria at an early date by Monod (1). He observed that when Escherichia coli encounters both glucose and lactose, for example, it metabolizes the glucose first and represses the use of lactose, resulting in biphasic growth (diauxie). This phenomenon is due to the repressive effect of glucose on the synthesis of enzymes required for the metabolism of other sugars. Later, the term catabolite repression was introduced as a general name for the glucose effect because compounds closely related to glucose elicited varying degrees of repression of glucose-sensitive enzymes, and the catabolites derived from the repressing compound were assumed to cause the glucose effect (2). Studies on catabolite repression up to the early 1970s brought about the discovery of a positive control system of transcription by cyclic AMP (cAMP) and the cyclic AMP receptor protein (CRP) (also called the catabolite gene activator protein, (CAP)) and led to the concept that catabolite repression is caused by reduction of the level of intracellular cAMP (3). We now know that multiple and different mechanisms are operating, depending on the growth conditions and the target operons, and that the cAMP-dependent mechanism is just one aspect of catabolite repression. Ironically, the preferential utilization of glucose over lactose, the prototype of catabolite repression, is not due to the reduction of the cAMP-CRP complex (4, 5).
 Sometimes the term catabolite repression has been used to describe the glucose effect that is independent of the operon-specific regulator, such as the Lac repressor. Here, we shall use it as a general name for the glucose effect as originally defined. According to this view, which is apparently generally accepted, catabolite repression includes all forms of the glucose effect, regardless of their mechanisms. We also concentrate on catabolite repression in E. coli. Although several mechanisms of glucose catabolite repression are now understood in E. coli, a common feature is that glucose causes catabolite repression ultimately by modulating the activity of transcription factors involved in the regulation of target operons.
1. cAMP–CRP Complex
The best characterized mechanism of catabolite repression involves the regulation of the intracellular concentration of the cAMP–CRP complex. It is well known that the presence of glucose in the growth medium lowers the intracellular cAMP level under certain conditions (3, 6). Cyclic AMP is synthesized from ATP by the enzyme adenylate cyclase. Although the mechanism of regulation of the cAMP level remains elusive, glucose is thought to decrease cAMP by decreasing the level of the phosphorylated form of enzyme IIA^Glc, which is involved in the activation of adenylate cyclase. IIA^Glc is one of the enzymes of the phosphoenolpyruvate-dependent carbohydrate phosphotransferase system (PTS) and is directly responsible for the active transport and phosphorylation of glucose (7). Recently, it was discovered that the concentration of CRP is also lowered by the presence of glucose and that this is an additional factor contributing to catabolite repression (8). The decreased CRP is a consequence of the complex autoregulation of expression of the crp gene (9). It should be noted that the reduction in the cAMP-CRP level by glucose is usually rather moderate (in the range of several-fold).
2. Inducer Exclusion
 The second mechanism of catabolite repression is inducer exclusion, by which glucose lowers the intracellular concentration of inducers necessary for the induction of catabolic operons (7). The target of glucose signaling in inducer exclusion is operon-specific regulators, such as the Lac repressor. The dephosphorylated enzyme IIA^Glc, which accumulates in the presence of glucose, binds to and inactivates (for example) the Lac permease, resulting in an increase of the active unliganded Lac repressor. Inducer exclusion is a mechanism by which glucose inhibits more strictly the expression of target operons.
3. Catabolite Repressor/Activator Protein
 The third mechanism of catabolite repression is mediated by the catabolite repressor/activator (Cra( protein, which acts as a global regulator of genes encoding enzymes of central carbohydrate metabolism (10). The unliganded form of Cra binds to the operator regions of target operons, causing either activation or inhibition of transcription. The presence of glucose or other PTS sugars produces glycolytic catabolites, such as fructose-1-phosphate, which bind to the Cra protein and cause it to dissociate from the target DNA, resulting in either catabolite repression or catabolite activation.
4. Relationships between the Various Mechanisms
While multiple mechanisms of catabolite repression have been identified in E. coli, their signaling pathways appear to be interrelated to each other. For example, the PTS plays a pivotal role in the regulation of the intracellular concentrations of cAMP, inducer, and glycolytic catabolites. In addition, it is particularly important to realize that the contribution of each mechanism varies, depending on growth conditions and the target genes. For example, the Cra-mediated mechanism may play no role in catabolite repression of the lac operon, because this operon is not under the control of Cra. An unexpected finding is that the presence of glucose in the lactose medium does not affect the intracellular cAMP level (4). This means that catabolite repression mediated by the reduction in cAMP never happens in glucose–lactose diauxie. The presence of unliganded Lac repressor through inducer exclusion is the principal mechanism for this historical phenomenon (4, 5). Figure 1 illustrates the present understanding of catabolite repression in the glucose–lactose system.

Figure 1. Mechanism of catabolite repression in the glucose-lactose system (4). When both lactose and glucose are present, glucose is transported and phosphorylated by the glucose PTS (IIA^Glc + IICB^Glc), increasing the concentration of the nonphosphorylated form of IIAGlc, which prevents the uptake of lactose by inhibiting the Lac permease activity. Thus, the concentration of lac inducer is very low in the presence of glucose, so the Lac repressor is active and represses transcription of the lac operon. It should be noted that glucose does not affect the binding of cAMP–CRP to the promoter, because the levels of cAMP and CRP are not reduced by the presence of glucose.
There are more stories yet to be elucidated before catabolite repression in E. coli is fully understood. Further diversity in the mechanism of catabolite repression is known in Gram-positive bacteria (11).
 
References
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