In the bacterium Bacillus subtilus , there are five genes coding for proline bio

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. (1 point) 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. (1 point) 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?

Explanation / Answer

a. In Bacillus subtilis, the proline biosynthesis genes are arranged in an operon. The five genes involved in the production of various enzyme of the proline biosynthesis comprise of the structural genes. These genese are under the control of same operator and promoter genes present adjacent to the cluster of five structural genes. So, all the five structural genes will be expressed with the help of a monocistronic mRNA (i.e. a single mRNA containing information of all the five genes and synthesized from the same promoter gene). If there is any mutation in any of the structural genes, there will be alterations in the DNA sequence of only that gene and hence will result in changes in the mRNA of only the region corresponding to that region without affecting the rest of the mRNA sequence. So, when such an mRNA is translated, there will be production of mutated (and hence non-funnctional) protein from the mutated region but from the rest of the normal portions of mRNA, normal and functional proteins will be synthesized. This situation can be compared with lacZ- mutation, where only one structural gene of lac operon (lacZ) is mutated and the other structural genes (lacY and lacA) remain functional. In such a situation the bacterium is unable to synthesize a functional b-galactosidase enzyme while it produces functional permease and acetyl transferase enzymes.

b. In this case, there is mutation in the end of the operon is resulting in loss of all five enzymes although there is no loss in the structural genes. This may due to the reason that the promoter sequence present at one end of the proline biosynthesis gene cluster undergoes mutation. A mutation in promoter gene will prevent binding of RNA polymerase to it, which in turn will prevent transcription of all the five structural genes. Hence, although there is no mutation in the structural genes, there will be no expression of these genes (due to mutation in promoter gene). This can be compared to lacP- mutants of lac operon. The lacP- mutants are unable to have normal binding of RNA polymerase and hence the expression of all the structural genes does not take place.

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