. A number of bacterial biosynthetic operons have been examined in Escherichia c

Question

. A number of bacterial biosynthetic operons have been examined in Escherichia coli and Salmonella typhimurium, many of which show attenuation very similar to that seen in the tryptophan system in E. coli. Below are partial amino acid sequences of leader peptides. For each, predict the amino acid that is likely to be synthesized by the operon for each attenuator element. Explain your reasoning. For leader peptide C, how would changing the histidine codons to alanine codons effect the regulation of the operon?

A. Met-Ser-His-Ile-Val-Arg-Phe-Thr-Gly-Leu-Leu-Leu-Leu-Asn-Ala-Phe-Ile-Val-Arg---
B. Met-Lys-Arg-Ile-Ser-Thr-Thr-Ile-Thr-Thr-Thr-Ile-Thr-Ile-Thr-Thr-Gly-Asn-Gly-Ala---
C. Met-Thr-Arg-Val-Gln-Phe-Lys-His-His-His-His-His-His-His-Pro-Asp---
D. Met-Lys-His-Ile-Pro-Phe-Phe-Phe-Ala-Phe-Phe-Phe-Thr-Phe-Pro---

Explanation / Answer

The Trp operon contains a leader peptide with a two tryptophan sequence, as well as four "regions" of the leader transript. Sequence 1 can bind to 2, 2 can bind to 3, and 3 to 4. When sequence 1 binds to 2, then sequence 3 binds to 4. If sequence 3 binds to sequence 4 then transcription terminates; if not, transcription continues. The two tryptophan sequence is what allows the polymerase to "sense" the levels of tryptophan in the cell. When tryptophan is low, the polymerase has to pause on that area (which is part of sequence 1) while tryptophan is "found". This pause physically prevents 1 from binding to 2, causing 2 to bind to 3, which allows transcription to continue, allowing more tryptophan to be made. So the way to predict the amino acid synthesized by an operon is to look for consecutive repeat amino acids in the leader sequence, and that will be the amino acid synthesized. So:

a. Leu
b. Thr
c. His
d. Phe

Changing the histidines to alanines in the leader sequence of C would cause the operon to be regulated by alanine concentration as opposed to histidine concentration, which ultimately means that histidine production would be regulated by alanine levels.

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