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Principles of Gene Regulation:- Many Prokaryotic Genes Are Clustered and Regulated in Operons

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p1085

2026-08-01

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Principles of Gene Regulation:- Many Prokaryotic Genes Are Clustered and Regulated in Operons

Bacteria have a simple general mechanism for coordinating the regulation of genes encoding products that participate in a set of related processes: these genes are clustered on the chromosome and are transcribed to gether. Many prokaryotic mRNAs are polycistronic— multiple genes on a single transcript—and the single promoter that initiates transcription of the cluster is the site of regulation for expression of all the genes in the cluster. The gene cluster and promoter, plus additional sequences that function together in regulation, are called an operon (Fig. 1). Operons that include two to six genes transcribed as a unit are common; some operons contain 20 or more genes. Many of the principles of prokaryotic gene expression were first defined by studies of lactose metabolism in E. coli, which can use lactose as its sole carbon source. In 1960, François Jacob and Jacques Monod published a short paper in the Proceedings of the French Academy of Sciences that described how two adjacent genes involved in lactose metabolism were coordinately regulated by a genetic element located at one end of the gene cluster. The genes were those for β-galactosidase, which cleaves lactose to galactose and glucose, and galactoside permease, which transports lactose into the cell (Fig. 2). The terms “operon” and “operator” were first introduced in this paper. With the operon model, gene regulation could, for the first time, be considered in molecular terms.

FIGURE 1 Representative prokaryotic operon. Genes A, B, and C are transcribed on one polycistronic mRNA. Typical regulatory sequences include binding sites for proteins that either activate or re press transcription from the promoter.

FIGURE 2 Lactose metabolism in E. coli. Uptake and metabolism of lactose require the activities of galactoside permease and β galactosidase. Conversion of lactose to allolactose by trans glycosylation is a minor reaction also catalyzed by β-galactosidase.