Question

The trp mRNA leader sequence (trpL) contains an attenuator region that is used as a regulatory mechanism for the production of tryptophan in the cell. When there are low levels of tryptophan present in the cell the ribosome stalls/pauses on the trp codons in the mRNA transcript, does this allow transcription to continue or cause it to terminate?  

Answer 1

Answer=When there are low levels of tryptophan present in the cell the ribosome stalls and stem loop 2-3 forms allowed continued transcription.

The mechanism of attenuation depend on the fact that translation in bacterial coupled with transcription,so ribosomes begin translating the 5' end of an mRNA while it is still being synthesized.Thus the rate of translation can affect the structure of growing RNA chain,which in turn determine whether further transcription can continue.

The critical region of Trp mRNA contain two adjacent tryptophan codon,so the rate of translation is highly dependent on tryptophan levels;this is the link between transcriptional attenuation and the availability of tryptophan.If tryptophan levels are low in the cell the ribosome stalls at this point and transcription of trp mRNA continues.If tryptophan is abundant translation continues and transcription is terminated.