Something must happen to induce transcription

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Unformatted text preview: and inhibiting transcription urn off • Positive control: a regulatory protein is an activator, stimulating transcription Operons can be either inducible or repressible • Inducible operons are those in which transcription is normally off (not taking place). • Something must happen to induce transcription. • Repressible operons are those in which transcription is normally on (taking place). these stay turned on • Something must happen to turn it off. • These possibilities can be combined in to generate different mechanisms useful for different types of genes. Some operons are inducible get repressor of dna Some operons are repressible Summary of different modes of control The E. coli lac operon • In 1961, Francois Jacob and Jacque Monod described the “operon model” for the genetic control of lactose metabolism in E. coli. • This and subsequent work established that the operon was the basic unit of transcriptional control in bacteria. • Used genetics to deduce the structure of the operon • analyzed the interactions of mutations that interfered with the normal regulation of lactose metabolism. The E. coli lac operon • Lactose • The major carbohydrate found in milk. • Metabolized by E. coli that reside in the mammalian gut. most adult humans cannot metabolize milk baseline = not able to metabolize milk Lactose metabolism presence of allolactose turns genes on The lac operon controls transcription of genes needed for lactose metabolism inducible negative operon A • !-galactosidase is encoded by the lacZ gene • Permease by the lacY gene B • Transacetylase (function in lactose metabolism unknown) by the lacA gene. • Common promoter lacP Text IPTG (isopropylthiogalactoside) also inactivates the repressor The lac operon controls transcription of genes needed in lactose metabolism A • Coordinated induction: If lactose is present and glucose is absent • the rate of transcription of these three genes increases ~1000X in 2-3 minutes. B Lac mutations and different components of the operon disrupted enzyme function G ROUP 1: ENZYME MUTATIONS 1. Enzyme/structural gene mutations are usually independent • Jacob and Monod isolated mutant strains that lost the ability to synthesize either !-galactosidase or permease • These mutations map to either the lacZ or the lacY genes and alter the amino acid sequences of the enzymes they encode. • Mutations were independent and usually affected only the product of the gene in which they occurred. Lac mutations and different components of the operon G ROUP 1: ENZYME MUTATIONS 1. Enzyme/...
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This document was uploaded on 02/25/2014.

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