Question

In the bacterium Bacillus subtilus, there are five genes coding for proline biosynthetic enzymes. They are located adjacent to one another on the chromosome. If excess proline is present in the medium, the synthesis of all five enzymes is coordinately repressed, whereas in the absence of proline, all five genes are coordinately expressed.

a. Most mutations in this region of the chromosome result in the loss of activity of only one of the enzymes. Explain how such a mutation would only effect the production of one enzyme. What is the counterpart of this type of mutation in the lac operon system?

b. Some mutations, mapping to one end of the cluster of proline biosynthesis genes, result in the loss of all five enzymes, even though none of the structural genes have been lost. Explain how such a mutation could affect all five genes. What is the counterpart of this type of mutation in the lac operon system?

Answer 1

A- In this, it might be possible that upon mutation one of the codons is mutated into stop codon which leads to prematurely stops the translation. Since all the protein coding region of their own Shine-Dalgarno sequence onto which ribosome binds so, the mutation doesn't affect the synthesis of other enzymes.

In case of Lac operon mutation might be in LacZ gene which produces Beta-galactosidase, and rest coding region will be normal

B- In each operon, there is promoter region on which RNA polymerase binds. Binding of RNA polymerase leads to transcription of the operon. If due to mutation there is the change in the sequence of the promoter region such that RNA polymerase is not able to bind to the promoter region. If there is no RNA polymerase binding there will not be any mRNA to synthesize all the enzymes.

In case of Lac operon, the same type of mutation is found when promoter region is mutated.