How would transcription of the E. coli trp operon be affected by the following m
ID: 226407 • Letter: H
Question
How would transcription of the E. coli trp operon be affected by the following manipulations of the leader region of the trp mRNA? a. Increasing the distance (number of bases) between the leader peptide gene and sequence 2 b. Increasing the distance between sequences 2 and 3 c. Removing sequence 4 d. Changing the two Trp codons in the leader peptide gene to His codons e. Eliminating the ribosome-binding site for the gene that encodes the leader peptide f. Changing several nucleotides in sequence 3 so that it can base-pair with sequence 4 but not with sequence 2Explanation / Answer
a. ANS: Less attenuation of transcription. The ribosome completing the translation of sequence 1 would no longer overlap and block sequence 2; sequence 2 would always be available to pair with sequence 3, preventing formation of the attenuator structure.
b. ANS: More attenuation of transcription. Sequence 2 would pair less efficiently with sequence 3; the attenuator structure would be formed more often, even when sequence 2 was not blocked by a ribosome.
c. ANS: No attenuation of transcription. The only regulation would be that afforded by the trp repressor.
d. ANS: Attenuation loses its sensitivity to Trp tRNA. It might become sensitive to His tRNA.
e. ANS: Attenuation would rarely, if ever, occur. Sequences 2 and 3 always block formation of the attenuator.
f. ANS: Constant attenuation of transcription. Attenuator always forms, regardless of the availability of tryptophan.
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