Answer:
C
Explanation:
Control over nitrogenase activity is accomplished by NH4+ prevents expression of the genes that encode nitrogenas.
Nitrogenase synthesis in free-living N2-fixing bacteria is universally under repressor control by ammonia. This, however, is not
the case for rhizobia. A need for N2 as inducer apparently does not
exist. The degree of ammonia repression is unusual in its strength.
Nitrogenase activity is at least 1O,OOO-fold lower in extracts of
NH4+-grown cells versus N2-grown cells.
