A number of bacterial biosynthetic operons have been examined in Escherichia col

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 peptide sequences, which encode by the leader RNA operon sequences, which in turn regulate the expression of operon through a process called translation or transcriptional attenuation are called leader peptides. Different operons have different types of leader peptide sequences. These sequences are usually present upstream to the structural genes of operon.

For the leader peptide-A (MSHIVRFTGLLLLNAFIVRGRPVGGIQH stop), the amino acid that is likely to be synthesized by the operon is “Leucine.” For the leader peptide-B (MKRISTTITTTITITTGNGAG stop), the amino acid that is likely to be synthesized by the operon is “Threonine.”

For the leader peptide-C (MTRVQFKHHHHHHHPD stop), the amino acid that is likely to be synthesized by the operon is “Histidine.” For the leader peptide-D (MKHIPFFFAFFFTFP stop), the amino acid that is likely to be synthesized by the operon is “Phenylalanine.”

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