Welcome to the ScienceDuo blog by Chris Wallis and Rhiannon Morris. Screeds on science and sanity from two people who understand neither.
Another short molecular biology snippet for you all! The Tryptophan operon is a super neat way of controlling gene expression. This is my favorite operon! Hope you enjoy it too!
The trp operon consists of 5 structural genes trpE, D, C, B and A, a control region that has a promoter and a leading peptide coding region that contains an attenuator (see diagram). Region 1 and 2 are able to pair with one another, region 2 and 3 can pair and likewise region 3 and 4 can pair. Pairing of region 3 and 4 forms the attenuator.
When trp levels are low: RNA pol begins transcribing RNA producing region 1 of the 5’ UTR. A ribosome attached to the 5’ UTR translates region 1 whilst region 2 is being transcribed. The ribosome stalls at the trp codons in region1 because trp is low and therefore region 2 is not covered when region 3 is transcribed. This means that when region 3 is transcribed it pairs with region 2. This prevents pairing between region 3 and 4, thus no attenuator is formed.
When Trp levels are high: RNA pol transcribes region 1 and then continues to region 2. Meanwhile a ribosome starts translating region 1 of the 5′ UTR- because Trp levels are high, the ribosome doesn’t stall and continues to translate region 2. This means that when region 3 is transcribed, region 2 is covered by the ribosome and these regions are unable to pair. Instead when region 4 is transcribed, it pairs with region 3 and the attenuator is formed. This prevents the translation of the structural genes in the operon.
Blog post by Rhee