The Bacterium Bacillus Subtilis Can Grow on Minimal Media with a Variety

question 29

Essay

The bacterium Bacillus subtilis can grow on minimal media with a variety of sugars as a carbon source. One such sugar is mannose, metabolized by the products of the man operon. Expression of the operon is controlled by a regulatory protein encoded in a separate gene, manR. Depending on conditions, the regulatory protein may bind at one of two sites in the operon, as follows:
(i) When mannose is absent from the cell, the regulatory protein is in a conformation called R1. R1 can bind specifically at an operator site manO. Binding of R1 at manO reduces transcription of the operon fourfold from a basal level of 20 units.
(ii) When mannose is present in the cell, it binds to the regulatory protein, causing it to undergo an allosteric transition from conformation R1 to a new conformation, called R2. R2 cannot bind at manO. However, R2 can bind specifically at a different site called the initiator, manI. Binding of R2 at manI increases transcription of the operon twofold from the basal level.
Mutations m1-m3, which affect expression of this operon, were identified. Each mutation affects only a single component of the operon. Levels of operon activity were measured in haploids. They were also measured in partial diploids with an F' carrying the wild-type alleles of all genes and regulatory elements described above. $\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\text { Operon activity }$
$\quad\quad\quad\quad\quad\quad\quad\quad\quad$ Haploids $\quad\quad\quad\quad\quad\quad\quad$ Partial Diploids
$\begin{array}{lcccc}&(-) \text { mannose }&(+) \text { mannose } &(-) \text { mannose }&(+) \text { mannose }\\\text { wild type } & 5 & 40 & 10 & 80 \\\mathrm{~m} 1 & 20 & 20 & 10 & 80 \\\mathrm{~m} 2 & 5 & 20 & 10 & 60 \\\mathrm{~m} 3 & 20 & 40 & 25 & 80\end{array}$ For each mutation, describe which component is affected. In addition, explain the observed activity in the haploid and partial diploid in each case.

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