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Because the structure and function of a cell are determined by the proteins it contains, the control of gene expression is a fundamental aspect of molecular cell biology. Most commonly, the “decision” to transcribe the gene encoding a particular protein is the major mechanism for controlling production of the encoded protein in a cell. By controlling transcription, a cell can regulate which proteins it produces and how rapidly they are synthesized. When transcription of a gene is repressed, the corresponding mRNA and encoded protein or proteins are synthesized at low rates. Conversely, when transcription of a gene is activated, both the mRNA and encoded protein or proteins are produced at much higher rates.

In most bacteria and other single-celled organisms, gene expression is highly regulated in order to adjust the cell’s enzymatic machinery and structural components to changes in the nutritional and physical environment. Thus at any given time, a bacterial cell normally synthesizes only those proteins that are required for its survival under the current conditions. Here we describe the basic features of transcriptional control in bacteria, using the lac operon and the glutamine synthetase gene in E. coli and the xpt-pbuX operon in Bacillus subtilis as our primary examples. Many of the same features are involved in eukaryotic transcriptional control, which will be the subject of the remainder of this chapter.

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