Genetics Lecture 30, 11/17: Chapter 16, trp Operon, Monod, Yanofsky and Bertrand

lab wed. pre lab write-up is water testing

For Powerpoint slides copy and paste this link in a new window: http://docs.google.com/Presentation?id=dhqwrndc_501cs7scpdq

1953- Monod and coworkers found an operon which is repressible. It was the trp operon.

Trp operon produces enzymes which help the cell synthesize tryptophan.

• (2:35) The presence of tryptophan turns off this operon (trp operon).
• Monod et. al. proposed that the repressor was normally inactive. It was turned on in the presence of tryptophan. This further led them to propose that the repressor had a co-repressor.
  
• What they figured out: the regulation was done with the assistance of constituitive mutants.
  
• 1st constituitive mutant was trpR- strain which produces the trpR protein that functions as a repressor protein
  
• (7:25) 2nd was a mutation in the operator region (analogous to lac O).
• In the absence of tryptophan the trpR repressor protein cannot bind the operator. Because the operator is unbound transcription takes place.
• (9:40) In the presence of tryptophan . . . the trpR repressor protein is made. Tryptophan binds to the trpR repressor. When this happens the repressor binds to the operator and prevents transcription.
  
• (12:20) 5'-UTR - an untranslated region. A sequence of DNA which is transcribed upstream of a gene but not copied. Typically they are involved in gene regulation.
  
• The untranslated region in the trp operon is 162 BP and it is transcribed but not translated. This untranslated region is studied by Yanofsky and Bertrand.

(17:15) Yanofsky and Bertrand (Slide Title: The role of the hairpin in the regulation of the trp operon).

• Irregardless of the presence or absence of tryptophan transcription of the trp operon begins.
• In the presence of tryptophan transcription ceases 140 BP into the 5' untranslated region. (140 of 162 are translated)
• In the absence of tryptophan transcription begins and continues to the end of the operon.
• There is a site between 115 and 140 nucleotides into the 5' untranslated region that serves as an attenuator sequence. (21:50) What happens is that this region can form hairpins in the 5'-UTR RNA.
  
• (23:30) In the presence of tryptophan the attenuator region forms two hairpins. This causes transcription to stop. In the absence of tryptophan the attenuator forms a single hairpin (called the anti terminator), transcription is able to complete.
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