Question

A friend tells you about a newly identified global regulatory system. In this system a number of genes (lphA, V, Q, Fetc.) are induced when there is limiting phosphorus in the environment. None of these genes are in an operon. In the course of characterizing this regulon, you isolate two significant mutants and become convinced (correctly) that the regulon is controlled by a DNA-binding protein.

The DNA binding protein that controls this regulon is a

A)	Activator that does not bind DNA in the presence of high phosphorus

B)	Repressor that does not bind DNA in the presence of high phosphorus

C)	Alternative sigma factor

D)	Repressor that binds DNA in the presence of high phosphorus

E)	Activator that binds DNA in the presence of high phosphorus

Answer 1

Correct answer is Option C - Alternative sigma factor

Explanation :

Bacteria contain a housekeeping sigma factor i.e. sigma70 which primarily transcribe most of the genes in normal conditions. When a bacteria encounters a different niche, say nutrient deficient environment or heat stress or presence of host,etc. , bacteria are observed to recruit an alternative sigma factor in response. These alternative sigma factors play essential role in adapting to the environmental stimuli. In case of nitrogen starved environment, sigma factor RpoN is activated and it induces the genes involved in nitrogen assimilation.