The E. coli trp operon encodes proteins involved in the biosynthesis of the amino acid tryptophan....

The E. coli trp operon encodes proteins involved in the biosynthesis of the amino acid tryptophan. Control of trp operon expression is mediated by two distinct regulatory mechanisms. Describe these two regulatory mechanisms in detail.

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Control of trp operon expression is mediated by two distinct regulatory mechanisms.

The trp operon is controlled by

1] Repressor protein binding to the operator region. - the trp repressor binds the operator region of the trp operon only when bound to tryptophan.

This makes tryptophan a co-repressor of the operon.

The trp-L gene encodes a non-functional leader peptide which contains several adjacent trp codons.

The structural genes of the operon responsible for tryptophan biosynthesis are trpE,D,C,B,A.

when tryptophan levels are high some binds to the repressor which then binds to operator region & inhibits transcription.

2] Translation-induced transcriptional attenuation =The attenuation region of the trp operon contains sequences that allow the resulting mRNA to form several different stem-loop structures.These regions are identified as 1-4

The stem loop that are significant as to whether transcription is attenuated or not are formed between regions 2 and 3 or between regions 3 and 4.

A] When tryptophan levels are high = there are plenty of charged trp-tRNAs available and ribosomes translating the leader peptide encoded by the trpLgene do not stall at the repeated trp codons in the leader peptide

under these conditions the ribosomes rapidly cover regions of 1 and 2 of the mRNA which allows the stem-loop composed of regions 3 and 4 to form.

The stem-loop formed by regions 3 and 4 results in a transcriptional termination structures & transcription of trp operon ceases,i.e. attenuated.

B] When tryptophan levels are low= the level of charged trp-tRNAs will also be low.This lead to a stalling of the ribosomes within the leader peptide when they encounter the trp codon repeats.The ribosome stalls over region 1 of the mRNA which allows stem-loop 2-3 to form and prevents the transcriptional termination atem-loop 3-4 from forming.

gladiator answered 1 year ago

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