Suppose there was a mutation in the leader peptide region 1 sequence of the trp

Question

Suppose there was a mutation in the leader peptide region 1 sequence of the trp operon, such that region 1 could not form a hairpin loop with region 2 (the trp codons are still present). Under which conditions would this mutation cause altered expression of the trp operon, relative to a non-mutated sequence? Choose all that apply.
[By altered expression, I mean that the mutated and non-mutated versions would be doing different things.]
1.
translation occurring, with plenty of tryptophan
2.
translation occurring, with a shortage of tryptophan
3.
no translation occurring

Explanation / Answer

The attenuator sequence at domain 1 contains instruction for peptide synthesis that requires tryptophans. A high level of tryptophan will permit ribosomes to translate the attenuator sequence domains 1 and 2, allowing domains 3 and 4 to form a hairpin structure, which results in termination of transcription of the trp operon. Since the protein coding genes are not transcribed due to rho independent termination, no tryptophan is synthesised.

In contrast, a low level of tryptophan means that the ribosome will stall at domain 1, causing the domains 2 and 3 to form a different hairpin structure that does not signal termination of transcription. Therefore the rest of the operon will be transcribed and translated, so that tryptophan can be produced. Thus, domain 4 is an attenuator. Without domain 4, translation can continue regardless of the level of tryptophan. The attenuator sequence has its codons translated into a leader peptide, but is not part of the trp operon gene sequence. The attenuator allows more time for the attenuator sequence domains to form loop structures, but does not produce a protein that is used in later tryptophan synthesis.

Hence, options 2 is applicable. option 3 is also applicable when the attenuator or mutation occurs in signal peptide domain 4.

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