13BIS101S2013GeneRegulLect13-1

Their monod approach to understanding this inductive

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Unformatted text preview: Operon Most of what we now know about gene regulation comes Most from studies of the lactose operon conducted by Jacob and Monod at the Pasteur Institute in Paris in the 1950s. Their Monod approach to understanding this inductive form of gene regulation was to destroy, by mutation, the genetic elements responsible for these effects. By treating E. coli cells with a chemical mutagen, they By E. isolated a number of mutants that fell into two general groups. Lactose deficient mutants unable to use lactose as a carbon source Lactose (lac-). ). Constitutive mutants that expressed β-galactosidase even in the Constitutive -galactosidase absence of lactose (i.e., loss of regulation). absence Constitutive mutants could subdivided into two categories: Constitutive repressor deficient and operator mutants. repressor operator BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 9 Genetics of the Lactose Operon Using these mutants, complementation analysis and Using complementation interrupted mating experiments revealed that the ability of E. coli to ferment lactose is encoded by three genes (lacZ, lacY coli lacZ, and lacI) and an intergenic region containing a promoter and and intergenic promoter (P) and operator (O). operator A collection of linked genes, all involved in the same collection metabolic process, is called an "operon." Although linked to Although the Z and Y genes, the "A" (transacetylase) is not required for lactose fermentation. Lactose Operon P I repressor repressor PO Z Y (A) β-galactosidase permease (transacetylase) BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 10 Lactose Operon: A polycistron Because the Z, Y and A genes are transcribed as one mRNA Because Z, molecule from one promoter, this region is called a polycistron and the mRNA is referred to as a polycistronic polycistron message. Ribosome binding sites (AGGA) precede the AUG codons for precede the Z, Y and A genes, so that three separate protein products Z, can be translated from this polycistronic mRNA. The Y gene encodes a permease that permits lactose to The permease enter the cell. The repressor is an allosteric protein whose allosteric interactions with the operator are controlled by an effector molecule called the inducer. inducer The inducer is allolactose, a metabolite of lactose, which The allolactose metabolite binds to the repressor and alters the repressor's affinity for the operator. BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 11 Operon Model Introduction to Lac Operon BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 12 Bacterial Genetics Bacterial Genetics: Sexual exchange of genetic...
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