Chapter Introduction

The Control of Gene Expression in Prokaryotes

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Bacteria respond to changes in their environments. A micrograph of the light organ of a newly hatched squid (Euprymna scolopes) is shown on the left. The light spots are due to colonies of the bacteria Vibrio fischeri that live symbiotically within these organs. These bacteria become luminescent when they reach an appropriately high density. The density is sensed by the circuit shown on the right in which each bacterium releases a small molecule into the environment. The molecule is subsequently taken up by other bacterial cells, which start a signaling cascade that stimulates the expression of specific genes.

OUTLINE

  1. Many DNA-Binding Proteins Recognize Specific DNA Sequences

  2. Prokaryotic DNA-Binding Proteins Bind Specifically to Regulatory Sites in Operons

  3. Regulatory Circuits Can Result in Switching Between Patterns of Gene Expression

  4. Gene Expression Can Be Controlled at Posttranscriptional Levels

Even simple prokaryotic cells must respond to changes in their metabolism or in their environments. Much of this response takes place through changes in gene expression. A gene is expressed when it is transcribed into RNA and, for most genes, translated into proteins. Genomes comprise thousands of genes. Some of these genes are expressed all the time. These genes are subject to constitutive expression. Many other genes are expressed only under some circumstances—that is, under a particular set of physiological conditions. These genes are subject to regulated expression. For example, the level of expression of some genes in bacteria may vary more than a 1000-fold in response to the supply of nutrients or to environmental challenges.

In this chapter, we will examine gene-regulation mechanisms in prokaryotes, particularly E. coli, because many of these processes were first discovered in this organism. In Chapter 32, we will turn to gene-regulation mechanisms in eukaryotes. We shall see both substantial similarities and fundamental differences in comparing gene-regulatory mechanisms of the two types of organisms.

How is gene expression controlled? Gene activity is controlled first and foremost at the level of transcription. Whether a gene is transcribed is determined largely by the interplay between specific DNA sequences and certain proteins that bind to these sequences. Most often, these proteins repress the expression of specific genes by blocking the access of RNA polymerase to their promoters. In some cases, however, the proteins can activate the expression of specific genes. We shall learn about several different strategies that allow the coordinated regulation of sets of genes. Some genes are also controlled at stages beyond the level of transcription and we shall examine several mechanisms at these stages. Finally, we will examine several important examples of how gene expression is regulated in response to changes in the concentrations of specific molecules in the environment of prokaryotic cells.

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