ls3prokyreg - Lecture 5: Prokaryotic Gene Regulation Why...

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Lecture 5: Prokaryotic Gene Regulation Why regulate gene expression? Cells don’t need to produce certain proteins all the time, and more importantly, it’s an energetically expensive process. The operon is one mechanism that bacteria use to control gene expression. Operon = A contiguous (side by side) arrangement of functionally related genes (Weaver p173), which enables coordinated regulation of their expression (and subsequent activity) A single promoter (transcription start site) drives expression of all the grouped genes, which results in a long mRNA encoding multiple genes, called a polycystronic message BUT!! Each gene encodes its own translational start site. This way, ribosomes bind to different sites for each gene on the polycystronic mRNA and produce separate proteins. This lecture goes over 3 examples that show how operons can be regulated. (1) lac operon (2) ara operon (3) trp operon Diauxic growth = refers to growth of bacteria that metabolizes 2 different, available sugars sequentially, which results in separate growth phases per sugar. Example: E. coli prefers glucose over lactose as a food source, and will only use lactose in the absence of glucose. “lag period” = time it takes for bacteria to produce enzymes needed to metabolize the 2 nd sugar Growth phase 1 Growth phase 2 lac operon encodes 3 genes lacZ, lacY, lacA required to metabolize lactose preferentially uses glucose both positive and negative control of gene expression negative control is the default mode; block RNA polymerase from transcribing lac operon genes unless needed (-)control when glucose available, regardless of lactose availability o lac repressor (encoded by lacI ) binds operator region(s) (+)control when [low/no glucose], lactose = only food source o CAP-cAMP complex (activator) binds CAP binding site
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lac operon Encodes a cluster of 3 genes involved in lactose metabolism ( lacZ, lacY, lacA ) lacI lacZ lacY lacA O 3 O 1 O 2 CAP RNA polymerase binding site binding site (promoter) Regulatory Binding sites (DNA sequences)
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This note was uploaded on 04/19/2011 for the course LS 2 taught by Professor Pires during the Spring '08 term at UCLA.

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ls3prokyreg - Lecture 5: Prokaryotic Gene Regulation Why...

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