Question

Ok, let's now think about the circumstances under which attenuation would occur, vs. the scenario that would promote read-through (i.e. attenuation does not occur). Describe what is happening in the figure below, and how this corresponds to levels of tryptophan in the .cell (2 pts). RNA polymerase DNA Completed MKAIFVLKG leader peptide MU096 Ribosome mRNA 5 Attenuator structure Trp codons

Answer 1

Attenuation occurs when tryptophan levels are high and act as a second negative feedback mechanism in the tryptophan operon reducing the expression of tryptophan operon by inhibiting the completion of transcription. In this scenario, ribosomes do not have to wait for a long time for trp tRNA leading to quick translation of the leader peptide.

Attenuation does not occur when tryptophan levels are low. In this scenario, ribosomes will pause at the trp codons causing slow translation of the leader peptide inhibiting the formation of stem and loop structure and thus allowing transcription to continue.

The leader sequence is one of the elements of tryptophan operon located 5' end of the trpE gene which through the process of attenuation controls the expression of tryptophan operon. The location of the attenuator is downstream of the operator prior to the first gene. When tryptophan levels are high, tryptophan tRNA levels are also high. mRNA quickly moves through the ribosome complex immediately after transcription causing quick translation. As translation occurs quickly, ribosome falls of the mRNA after translating the leader peptide. This leads to the formation of stem and loop structure which makes RNA polymerase to detach ending the process of transcription.