Regulation of Gene Expression

Having established how DNA is organised into genes, how those genes store hereditary information, and how that information is expressed as protein, the chapter turns to a more fundamental question: how is gene expression itself switched on and off? There is strong experimental evidence that genes are not simply 'always on', but can instead be actively turned on or off depending on the cell's needs, and this regulatory phenomenon has been most thoroughly studied in the bacterium E. coli. Gene expression can, in principle, be controlled at several different points, transcriptionally, post-transcriptionally, or translationally, but this section focuses specifically on regulation at the transcriptional level, where small extracellular or intracellular metabolites commonly act as the trigger that either switches gene expression on or shuts it off. Clusters of functionally related genes that are transcribed together, as a single mRNA molecule, are collectively called an operon; in E. coli alone, roughly 260 genes are organised into about 75 separate operons. Every operon is built from the same basic architecture: one or more structural genes, which actually code for the proteins, rRNA or tRNA the cell needs; a promoter, the DNA signal sequence that RNA polymerase binds to before it can begin transcribing the structural genes; and an operator, a DNA sequence positioned between the promoter and the structural genes, to which a regulatory repressor protein specifically binds. The lac (lactose) operon of E. coli, first explained by Jacob and Monod's now-classical model of gene regulation, is the standard textbook example. Digesting lactose for energy requires three separate enzymes: permease, which allows lactose to enter the cell in the first place; beta-galactosidase, which hydrolyses lactose into its two component sugars, glucose and galactose; and transacetylase, which transfers an acetyl group from acetyl-CoA onto beta-galactosidase. The lac operon consists of one regulatory gene, called the 'i' gene (for inhibitor), a promoter site (p) and an operator site (o), together with three structural genes, lac z (coding for beta-galactosidase), lac y (coding for permease) and lac a (coding for transacetylase), that are all transcribed together as a single polycistronic mRNA under the control of one shared promoter and regulatory gene. When the cell is using its normal, preferred energy source, glucose, the regulatory 'i' gene is transcribed and translated into a repressor protein, which binds directly to the operator region and physically prevents RNA polymerase from transcribing the structural genes, so none of the three lactose-digesting enzymes is produced. When lactose is available instead of glucose, permease allows a small initial amount of lactose to enter the cell, where it acts as an inducer, either directly or, more precisely, once converted to allolactose: the inducer binds the repressor protein and inactivates it, so it can no longer bind the operator, which frees RNA polymer …