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Chapter 22: Problem 13

In the operons that contain genes for isoleucine biosynthesis, the leader regions that precede the genes contain multiple codons that specify not only isoleucine but valine and leucine as well. Suggest a reason why this is so.

Short Answer

Expert verified
The leader region of an operon for isoleucine biosynthesis also contains codons for valine and leucine to effectively coordinate the production and regulation of these structurally similar and functionally related branched-chain amino acids.

Step by step solution

01

Understand Codon and Operon Function

Codons are segments of DNA that code for specific amino acids. The leader region in an operon is a segment of DNA that precedes the set of genes that code for specific proteins or processes. In this case, we consider an operon that codes for isoleucine biosynthesis, but it鈥檚 noted that the leader region also contains codons for valine and leucine. This implies that these amino acids are somehow related in their biosynthesis or regulatory processes.
02

Consider the Amino Acids

Isoleucine, valine and leucine are all branched-chain amino acids. The biosynthesis of these amino acids is related since they all begin from the same precursor: Pyruvate. Therefore, the leader region of the operon might contain codons for all the three amino acids because their production pathways are linked.
03

Regulation of Operon

Operons are regulated as a unit. When conditions call for abundant quantities of isoleucine, it鈥檚 likely necessary to also increase production amounts of valine and leucine due to their similar structure and biological function. The regulation (either up-regulation to increase production or down-regulation to decrease production) can be done more effectively when the genes are co-located in the genome. This is likely the reason why gene clusters for their synthesis are found in the same operon.

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Most popular questions from this chapter

Transfer RNAs are absolutely essential for polypeptide synthesis. After reviewing the material in this chapter, name five different cellular components that can bind to (interact with) tRNA molecules.

A prokaryotic mRNA may contain many AUG codons. How does the ribosome distinguish AUG codons specifying initiation from AUG codons specifying internal methionine?

The standard genetic code is read in codons that are three nucleotides long. How many potential reading frames are there on a single piece of double- stranded DNA? If instead the genetic code was read in codons that were four nucleotides long, how many reading frames would there be on the same piece of double-stranded DNA?

Bacterial genomes usually contain multiple copies of the genes for rRNA. These are transcribed very efficiently in order to produce large amounts of rRNA for assembly into ribosomes. In contrast, the genes that encode ribosomal proteins are present only as single copies. Explain the difference in the number of copies of rRNA and ribosomal protein genes.

On rare occasions, the translation machinery encounters a codon that cannot be quickly interpreted because of the lack of a particular tRNA or release factor. In these cases, the ribosome may pause and then shift by a single nucleotide and begin translating a different reading frame. Such an occurrence is known as translational frameshifting. The E. coli release factor RF-2, which is translated from mRNA that contains an internal UGA stop codon, is produced by translational frameshifting. Explain how this phenomenon might regulate RF- 2 production.
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