Lac2 - Biol 213 Genetics: Friday, October 27, 2000 Lac...

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Regulation of gene expression (prokaryote) II - 1 Biol 213 Genetics: Friday, October 27, 2000 Lac Operon (Part II) Outline I. THE OPERATOR AND THE NATURE OF REPRESSION II. THE MOLECULAR NATURE OF BINDING SITES: the promoter and the operator pp.341-343 III. COMPLEMENTATION TESTS OF ELEMENTS OF LAC REGULATION IV. POSITIVE REGULATION AND CRP, pp.404-405 A. Symbolic representation of a cellular state: cAMP B. Model of positive regulation: CRP protein C. Experimental test of positive regulation of the lac operon through CRP and cAMP V. WHAT CAN THE LAC OPERON TEACH US? A. Integration of Signals B. Types of regulation I. The operator and the nature of repression We left each other Wednesday with visions of the lac operon (e.g. Fig. 15-4) dancing in our heads, a picture that represents the product of the 20 years Jacques Monod had pursued the problem of how the expression of ß-galactosidase is regulated. That picture depicts a repressor protein as a critical element of the regulation, the element that mediates the response of lacZ to the presence or absence of lactose. However, by the end of the 1950's, when much of this picture was coming together, it was by no means clear how ß-galactosidase expression was regulated. Monod had started off with a completely different notion. He figured that since lactose increases ß-galactosidase activity and lacI is required for proper regulation, lactose must act through the LacI product to induce expression (which is why he named the gene lacI -- I for i nduction). We know now that the protein encoded by lacI is a repressor, not an inducer and slowly Monod came to accept that possibility. But how to put the notion of a repressor to the test? Monod had the following test in mind. He Most students find the lac operon complicated and confusing. If it weren't, it would not be able to perform all the functions that E. coli demands of it. The lac operon is useful to us as well, because it contains within it the most important themes of gene regulation. We are focusing our attention on the lac operon, rather than surveying many genes in a more superficial way, so that we can gain a deep understanding of some strategies of regulation that can be applied to a large number of genes. You will find that the concepts learned through study of the lac operon will appear again as we consider eukaryotic gene regulation and the control over multicellular development.
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Regulation of gene expression (prokaryote) II - 2 reasoned that if a repressor existed, it must repress something, and it should be possible to find a mutation in that something. Just as you can ruin the regulation of your front door by mutating either the lock or the key, so you should be able to ruin the regulation of the lac operon by mutating either the repressor or the object of repression.
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This note was uploaded on 11/07/2011 for the course BIO 325 taught by Professor Saxena during the Summer '08 term at University of Texas at Austin.

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Lac2 - Biol 213 Genetics: Friday, October 27, 2000 Lac...

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