Regulation of Bacterial Gene Activity Help
Regulation of Bacterial Gene Activity
Within any cell, not all genes are active at the same time. Some gene products need to be continuously synthesized, whereas others are necessary only during certain phases of the life cycle or in particular environments. Even when genes are "turned on," the quantity of proteins they specify may need to be controlled. Therefore, the activity of virtually all genes needs to be regulated to make the most efficient use of the energy available to the cell. These mechanisms may act at one or more levels, such as at the level of the gene by controlling the timing and/or rate of transcription. Other control mechanisms may operate during or after translation.
The transcriptional activity of genes may be unregulated if their products are needed regardless of environmental conditions. Such products are said to be synthesized constitutively. The quantity of products from constitutively expressed genes can vary, however, depending upon the relative affinities of their promoters for RNA polymerase. Proteins that are required only under certain conditions, are usually governed by one or more regulatory proteins. Regulatory proteins usually do not have an enzymatic function, but instead interact with the DNA in or near the promoter of a gene to regulate transcription.
There are two basic types of regulatory proteins: repressors and activators. A repressor protein binds to a site called the operator within an operon. The attachment of a repressor protein to an operator prevents transcription of the structural genes in the same operon. A gene with this form of regulation is said to be under negative control. Proteins required for the expression of an operon are called activators. They may bind to initiator or activator sites that are located within an operon's promoter or, in the case of enhancer sites, they may bind at sequences far from the operon. When the binding of a regulatory protein to an initiator or enhancer site stimulates transcription of structural genes in the operon, a positive control mechanism is said to be at work.
The stimuli to which regulated genes respond may vary from relatively small molecules (e.g., sugars, amino acids) to relatively large substances (e.g., in eukaryotes, a complex of a steroid hormone and its protein receptor). A substance that turns on gene transcription is referred to as an inducer, whereas a substance that turns transcription off is said to be a corepressor. Inducible genes are usually involved in catabolic (degradative) reactions, as in the breakdown of a polysaccharide into simple sugars. Repressible genes are usually involved in anabolic (synthetic) reactions, as in the construction of amino acids from simpler precursors. Thus, there are two main types of transcriptional controls: (1) negative and (2) positive. Negative control can either be inducible or repressible and positive control is known only to be inducible. In addition, genes may be regulated by global controls or by posttranslational mechanisms discussed in sections 4 and 5 in the following text.
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