Chapter11_SSM - 65781_CH11_222_239.qxd 1:01 PM Page 222...

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Chapter 11: Molecular Mechanisms of Gene Regulation Chapter Summary The processes of gene regulation generally differ between prokaryotes and eukaryotes, but transcriptional regulation plays a key role in both types of organisms. Transcriptional regulation may be either negative (“on” unless specifically turned “off”) or positive (“off” unless specifically turned “on”). A negative regu- latory system may be inducible (when the repressor protein is inactivated by combining with the inducer) or repressible (when the repressor is formed by the combination of aporepressor protein and the co-repressor). Positively regulated systems are typically controlled by transcriptional activator proteins. Lactose metabolism in E. coli is a classic example of an inducible system. When lactose is present, the genes for the enzymes required to metabolize lactose are transcribed; when lactose is absent, transcription is repressed. Two proteins needed to utilize lactose (permease, required for the entry of lactose into the cell, and β -galactosidase, the enzyme that cleaves lactose) are tightly linked and transcribed into a single poly- cistronic mRNA molecule, lac mRNA. Immediately adjacent to the promoter for lac mRNA is a regulatory sequence of bases called the operator. A repressor protein, the product of the lacI gene, binds with the operator and prevents transcription. The inducer binds to the repressor and inactivates it. Therefore, in the absence of lactose the permease and β -galactosidase genes are not transcribed, but in the presence of lactose there is no active repressor, and the lac promoter becomes accessible to RNA polymerase. The operator, the promoter, and the structural genes are adjacent to one another and together constitute the lac operon. Repressor mutations have been isolated that inactivate the repressor protein, and operator mutations are known that prevent recognition of the operator by an active repressor; such mutations cause continuous production of lac mRNA and are said to be constitutive. The lac operon as well as other sugar-utilization operons are also subject to positive regulation through binding with the cAMP–CRP complex. (The cAMP stands for cyclic AMP, and CRP for cyclic AMP receptor protein .) When glucose is present in the growth medium, the level of cAMP is low, and not enough cAMP–CRP is formed to allow transcription of the operons. Only when glucose is absent is the concentration of cAMP sufficient to produce enough cAMP–CRP to make transcription possible. 222
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223 Many biosynthetic enzyme systems exemplify the repressible type of negative transcriptional control. In the trp operon in E. coli , transcription of the operon is controlled by the concentration of tryptophan in the growth medium. When excess tryptophan is present, it binds with the trp aporepressor to form the active repressor that prevents transcription. Regulation of the trp operon is also subject to fine tuning by attenua-
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Chapter11_SSM - 65781_CH11_222_239.qxd 1:01 PM Page 222...

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