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Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription
The Logic of Prokaryotic Gene Regulation

Bacteria have a profound need to regulate the expression of their genes. Enzymes taking part in sugar metabolism provide an example. Metabolic enzymes are required to break down different carbon sources to yield energy. However, there are many different types of compounds that bacteria could use as carbon sources, including sugars such as lactose, glucose, galactose, and xylose. A different enzyme is required to allow each of these sugars to enter the cell. Further, a different set of enzymes is required to break down each of these sugars. If a cell were to simultaneously synthesize all the enzymes that it might possibly need, it would cost the cell much more energy to produce the enzymes than it could ever derive from breaking down any of the prospective carbon sources. The cell has devised mechanisms to shut down (repress) transcription of all the genes encoding enzymes that are not needed at a given time, and to turn on (activate) those genes encoding enzymes that are required. For example, if lactose is the primary carbon source in the environment, the shutdown of transcription of the relevant genes ensures that no enzymes are present for the import and metabolism of glucose, galactose, xylose, and other sugars. Conversely, the initiation or transcription of the appropriate genes ensures that the enzymes for lactose import and metabolism are activated. Such selective repression and activation mechanisms must fulfill two criteria:
They must have the ability to reversibly turn on or switch off transcription of each specific gene or group of genes.
They must be responsive to environmental conditions in which transcription of the relevant gene(s) should be activated or repressed.

Let's preview the current model for transcriptional regulation and then use a well-understood examplethe regulation of the genes involved in the metabolism of the sugar lactoseto examine it in detail.

© 1999 by W. H. Freeman and Company. All rights reserved.