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Revised Biology Questions & Answers
Q. Which of the following occurs as a result of an abundance of tryptophan in E. coli?
- The 5 trp genes (TrpA – TrpE) are not transcribed.
- Stalling of the ribosome at trp codons in the leader sequence
- The 5 trp genes (TrpA – TrpE) are transcribed, but not translated.
- The leader sequence is not transcribed.
ANSWER: 1. The 5 trp genes (TrpA – TrpE) are not transcribed.
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ANSWER IN DETAIL
The bacterium Escherichia coli take tryptophan (a type of amino acid) from the environment to build proteins for its body. But if it is absent in the environment then E. coli can produce its own tryptophan by the five genes present in it.
These genes encode enzymes for the transcription process. These cluster of 5 genes, trp operon has a promoter and operator site collectively forms trp operon.
The turning on and off operon is regulated by a protein called trp repressor. This protein act both as a switch and a sensor. It senses if the tryptophan is already present at excessive quantity.
Then the tryptophan binds to the trp repressor protein. This leads to the change in the shape of a protein. Now, the molecule is said to be in an active state.
The repressor protein is known as a corepressor in the active state. This corepressor gets attached to the operator site.
Due to its attachment, the RNA polymerase is not converted into mRNA and the transcription process stops.
In case, the tryptophan is present in lower concentration then it does not bind with the repressor protein.
Thus, it stays in the inactive form and does not attach to the operator site and hence the transcription of RNA polymerase takes place.
Along with this, if the tryptophan is present in a high amount then a process called attenuation takes place.
This rather than blocking the initiation of the process prevents the completion of the process. Thus, no unnecessary biosynthetic enzyme production takes place.
So, if the tryptophan is present in excess, it will give negative feedback to the genes involved in the tryptophan synthesis and the attenuation process starts leading to the transcription process to stop.
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