GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

20. METABOLIC REGULATION

20.3. MECHANISMS OF ENZYME SYNTHESIS (INDUCTION AND REPRESSION)

20.3.1. Induction of the Lactose Operon

The mechanisms of Induction and Repression have been studied in the greatest detail during the synthesis of lactose catabolic Enzymes in E. coli (Fig. 20.6).

Three genes encoding β-galactosidase, permease, and transacetylase (structural genes) are located adjacently on the chromosome. The DNA regions to which regulatory Proteins bind are directly adjacent to the structural genes. These regions are called the promoter (P) and the operator (O). The promoter is a base sequence recognized by DNA-dependent RNA polymerase. The promoter serves as the binding site for RNA polymerase, marking the initiation of METABOLISM/31.html">Transcription. The operator is a nucleotide sequence located between the promoter and the structural genes. The operator interacts with a regulatory repressor protein, which determines whether transcription will be repressed or not. The promoter, operator, and structural genes together form an Operon. An operon is defined as a group of functionally related genes.

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Fig. 20.6. Model of lactose operon regulation

Regulatory genes (i and r; located to the left of the promoter in Fig. 20.6) are responsible for the synthesis of regulatory proteins. They are not necessarily located adjacent to their corresponding operon. Regulatory genes are typically constitutive. The termination of mRNA synthesis is mediated by the terminator (t), a specific DNA region located near the end of the operon. This entire Structure is called the lactose operon. Let us examine how the lactose operon Functions. Figure 20.6 illustrates three possible scenarios: without an inducer (lactose); with an inducer (lactose); and catabolite repression (in the presence of two substrates—lactose and glucose).

In the first case (Fig. 20.6, B) (in the absence of lactose), the repressor binds to the operator and prevents transcription, meaning the operator region is blocked by the repressor.

When lactose enters The Cell, it is converted by β-galactosidase into allolactose, which acts as an internal inducer. This internal inducer binds to the repressor, causing Conformational Changes in the repressor such that it can no longer bind to the operator (Fig. 20.6, A). As a result, the operon remains free, making transcription possible. This is known as negative regulation. In negative regulation, the repressor must be displaced by the inducer for transcription to be initiated.

There is also positive regulation, where a necessary condition for transcription is the interaction of the operon with an activator, a second regulatory protein known as CAP or CRP (Fig. 20.6, A). CAP (catabolite activator protein) and CRP (cyclic AMP receptor protein) are synonyms. Transcription can only occur when the CAP protein is bound to the promoter. Such binding of CAP to the promoter is a prerequisite for the attachment of RNA polymerase to DNA. However, CAP, in turn, can bind to the promoter only if cyclic AMP (cAMP) is present in the cell at a sufficiently high concentration.

In the third case (catabolite Repression of the lactose operon; Fig. 20.6, C), when both lactose and glucose are present in the medium, the synthesis of lactose operon enzymes is repressed. This effect of glucose is due to the fact that its presence results in a very low concentration of cyclic AMP. Under these conditions, CAP cannot bind to the promoter, RNA polymerase fails to attach to the DNA, and transcription does not occur. Consistent with this mechanism, catabolite repression of lactose operon enzyme synthesis in the presence of glucose can be relieved by adding high concentrations of cyclic AMP to the medium.