For legend see opposite page
Figure 2-6 Overview of translation in which mRNA serves as the template for the assembly of amino acids into polypeptides. The three steps include initiation (A), elongation (B and C), and termination (not shown).
(i.e., are not transcribed and therefore are not expressed) in the presence of those products. This strategy avoids waste and overproduction of products that are already present in sufficient supply. In this system the product acts as a co-repressor that forms a complex with a repressor molecule. In the absence of co-repressor product (i.e., gene product) transcription occurs (Figure 2-7, A). When present in sufficient quantity, the product forms a complex with the repressor. The complex then binds to a specific base region of the gene sequence known as the operator region (see Figure 2-7, B). This binding blocks RNA polymerase progression from the promoter sequence and inhibits transcription. As the supply of product (co-repressor) dwindles, an insufficient amount remains to form a complex with the repressor, the operator region is no longer bound, and transcription of the genes for the anabolic enzymes will commence and continue until a sufficient supply of end product is again available.
In contrast to repression, genes that encode catabolic enzymes are usually induced, that is, transcription only occurs when the substrate to be degraded by enzymatic action is present. Production of degradative enzymes in the absence of substrates would be a waste of cellular energy and resources. When the substrate is absent in an inducible system, a repressor binds to the operator sequence of the DNA and blocks transcription of the genes for the degradative enzymes (see Figure 2-7, C). In the presence of an inducer, which often is the target substrate for degradation, a complex is formed between inducer and repressor that results in the release of the repressor from the operator site, thus allowing transcription of the genes encoding for the appropriate catabolic enzymes (see Figure 2-7, D).
Certain genes are not regulated, that is, they are not under the control of inducers or repressors. These genes are referred to as constitutive. Because they usually encode for products that are essential for viability under almost all growth and environmental conditions, these genes are continuously expressed. Also, not all regulation occurs at the genetic level (i.e., the level of transcription). For example, the production of some enzymes may be controlled at the protein synthesis (i.e., translational) level. The activities of other enzymes that have already been synthesized may be regulated at a posttranslational level, that is, certain catabolic or anabolic metabolites may directly interact
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