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 functioning unit of genomic DNA that contains cluster of genes under the control of single promoter is called operon. The genes in the operon are transcribed together to form an mRNA. The produced mRNA is either translated together in cytoplasm or undergoes splicing to form monocistronic mRNA.

In the given case, the presence of histidine codon indicates the operon as histidine operon. Similar to tryptophan operon, where tryptophan acts as corepressor in the given condition histidine acts as corepressor. The replacement of histidine codon with alanine codon will lead to a condition where cell will be unable to produce the histidine.

The histidine operon in E.coli is about 210 nucleotides length. In the leader sequence, the multiple codons for the synthesized amino acid are present. It is about 7/14 codons of histidine operon.

Thus, among the given options C is correct answer.

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