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slides 9 - #9 Prokaryotic transcription (II) 1. 2. 3. 4....

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#9 Prokaryotic transcription (II) 1. Gene and Operon 2. lac operon - protein regulators 3. trp operon - protein and RNA regulators 4. Riboswitch - an allosteric RNA regulator For chapter 7, section 7.2 is not covered
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Recall a slide from Lecture #2 “How proteins work?” 1. Proteins work by (1) binding to other molecules, including small ones such as ions or sugars, and large ones such as proteins, DNA, and RNA, and/or (2) catalyzing chemical reactions. 2. Protein function can be measured by affinity (binding strength), specificity (binding preference), and velocity (speed of reactions). Point #2 implies that proteins do not behave black-white. Proteins behave statistically and dynamically . For example, when we say protein X binds to DNA Y in a cell, you may assume: (1) not every X binds to Y, (2) not every Y is bound by X at every moment, (3) X may fall off from Y and it can go back to bind to it again. A statistical and dynamic view of molecule interaction
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Operon is defined by Weaver and others as " a group of genes coordinately regulated by one operator ”. Because we usually consider promoter as the integral part of a gene, and that operator is simply a regulatory DNA sequence that may also be defined as part of a gene, I prefer to define operon as " a bacterial gene encoding more than one protein ". Prokaryotic genes are often polycistronic : —one promoter directs transcription of a mRNA that can encodes more than one protein Eukaryotic genes are always monocistronic : —one promoter directs transcription of a mRNA that encodes only one protein 1. Gene and operon
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Polycistronic and monocistronic genes Polycistronic: one gene encodes one mRNA that can be translated to more than one polypeptide. Monocistronic: One gene encodes one mRNA translated to only one polypeptide. Why?
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Why prokaryotic genes are polycistronic, but eukaryotes monocistronic? 1. Proteins encoded by a prokaryotic polycistronic gene are usually all involved in the same biochemical pathway. This allows simple regulation of the whole pathway, favored by fast growing bacterial cells. Eukaryotes prefer to do things in more sophisticated ways to achieve better defined control of the process, and they don’t mind to be a bit slower because they usually live longer. 2. One mRNA of a prokaryotic gene has many ribosome- binding (=translation initiation) sites. One mRNA of an eukaryotic gene has only one translation start site. So, polycistronic gene also translates faster
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Evolution of operons and genes A polycistronic gene encoding 5 enzymes used for tryptophan synthesis in prokaryotes (e.g. E. coli) became 5 monocistronic genes in eukaryotes (e.g. yeast) encoding 5 enzymes with the similar function. It might occur through DNA recombination (cut-and-paste) during evolution
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LacZ β -galactosidase , it splits lactose to glucose and galactose (human lactase is also a β -galactosidase) LacY lactose permease , it imports lactose into the cell. LacA
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This note was uploaded on 02/20/2009 for the course LS 3 taught by Professor Lin during the Spring '06 term at UCLA.

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slides 9 - #9 Prokaryotic transcription (II) 1. 2. 3. 4....

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