If the regulator protein of the arginine regulon is mutated such that it can not bind arginine, which of the following describes the phenotype of the bacterium with such a mutation? a) inducible
b) repressible
c) uninducible
d) constitutive
e) none of these

The answer is a repressive mutation (the term is similar to b option).

Explanation:

In bacteria, the arginine regulon consists of a dozen genes required during the transport and biosynthesis of arginine (arg genes), these genes are controlled as a single unit. In prokaryotes, the concept of regulon is similar to the operon; however, an operon is composed of a group of genes linearly localized on bacterial chromosome, while a regulon can be formed by genes localized in different chromosome regions. Moreover, in eukaryotes, this concept is also used in order to indicate a set of linked genes that are regulated by the same regulatory gene sequence. A repressive mutation in a positive regulator of the arg regulon may trigger the inhibition of arg genes by affecting their ability to bind arginine.

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