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lecture_22_cont - Lecture21cont Polarity Defined:...

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Lecture 21 cont.
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Polarity Defined: the premature translational termination of an upstream gene causes the premature transcriptional termination of the downstream gene: P P
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Why? Look for mutants that overcome polarity System: Lac system, with a known polar mutation in lacZ Select for LacY + and screen for LacZ lacZ lacY lacA X LacZ LacY LacA
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Mutants were designated sup, for suppressor What types of mutants do you think were obtained? Rho, why?
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Model When translation stops, transcription continues Ribosomes are not available to “hide” rut sites Exposed rut sites allow Rho to bind Leading to premature termination
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Explains a mystery lac Z Here is lacZ and each line represents a translational stop Position of stop % LacY + or lacY expression 100%
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The closer the stop is to the end of the gene, the more down stream expression is observed. Polarity decreases as the stop moves to the end of the gene.
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Attenuation Control transcription by controlling a termination event Under one condition termination is enhanced, under a second condition termination is inhibited.
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