Catabolite repression

Catabolite repression is a mechanism for gene regulation. It is used to inhibit the production of certain catabolic enzymes of a cell, as long as a generally energetically favorable substrate - typically a different carbon source - an alternative catabolic pathway is present in a significant amount.

Principle

In a catabolite repression by the original assumption causes a catabolite of glucose metabolism, inhibition of gene expression specific for the metabolic pathway related genes, such as the lac operon. However, only after the designation of the observed effect was revealed that not a catabolite of glucose inhibits gene expression, but the second messenger cAMP, which is formed when glucose deficiency and activator to the catabolite protein (CAP, cAMP response protein, CRP) binds. After cAMP binding dimerizes CAP and acts as an activator of gene expression. Glucose leads to decreased cAMP concentration. The catabolite repression is thus an activation of gene expression in the absence of the substrate. Incidentally has the lac operon and a lac repressor, which releases the promoter upon binding of lactose. In contrast to the catabolite feedback inhibition is a product inhibition.

Was first demonstrated this mechanism using the example of glucose, which is why he was formerly called glucose effect. However, other carbon sources may be responsible for catabolite repression.

It was noticed that certain metabolic pathways of a bacterium (ie, the synthesis of certain enzymes failed to ) was inhibited when glucose in the medium was present. In the absence of or after consumption of glucose, the previously repressed enzymes were expressed. This will contain the growth of Escherichia coli in a medium containing glucose and lactose, especially clear. First, the culture grows relatively quickly and consumed the glucose, it is followed by a brief period of no growth ( lag phase or lag phase ), and then the growth continues under Lactoseverbrauch, but more slowly. During growth on lactose, the lactose operon is induced when E. coli. First, however, this induction was omitted in favor of the utilization of glucose present in the medium. The short lag phase between full utilization of glucose, and the beginning of the lactose thus describes the period in which the lactose operon was induced, and the enzymes were lactoseverwertenden formed. Such growth behavior with two separate exponential phase is also called diauxic growth or short diauxie.

The mechanism underlying this phenomenon is based is the catabolite repression. Glucose inhibits the adenylate cyclase, an enzyme which is from ATP cyclic adenosine monophosphate (cAMP ) is synthesized. cAMP is a universal intracellular messenger or signaling molecule ( second messenger ) and regulates many cellular processes. An important function of cAMP in the context described here is called the binding to CAP ( catabolite activator protein ). In katabolitreprimierten genes, RNA polymerase can only bind to DNA and initiate transcription, if previously bound to the DNA CAP. This accumulation can only take place in a previous binding to cAMP. Causing a catabolite a decrease in the intracellular cAMP concentration, so you can cap-dependent genes are not transcribed and enzymes are not formed. The lactose operon is subject to this same control mechanism, but it is by far not the only one. On the one hand be controlled by many of the operon of the cAMP levels, on the other hand operon partially separate regulators have (in the case of the lac operon, lactose and the lac repressor ), this simple regulation system enables the modulation of cellular metabolism.