Role of glutamine synthetase in nitrogen metabolite repression in Aspergillus nidulans

Glutamine synthetase (GS), EC 6.3.1.2, is a central enzyme in the assimilat ion of nitrogen and the biosynthesis of glutamine. We have isolated the Asp ergillus nidulans glnA gene encoding GS and have shown that glnA encodes a highly expressed but not highly regulated mRNA. Inactivation of glnA result s in an absolute glutamine requirement, indicating that GS is responsible f or the synthesis of this essential amino acid. Even when supplemented with high levels of glutamine, strains lacking a functional glnA gene have an in hibited morphology, and a wide range of compounds have been shown to interf ere with repair of the glutamine auxotrophy. Heterologous expression of the prokaryotic Anabaena glnA gene from the A. nidulans alcA promoter allowed full complementation of the A. nidulans glnA Delta mutation. However, the A . nidulans fluG gene, which encodes a protein with similarity to prokaryoti c GS, did not replace A. nidulans glnA function when similarly expressed. O ur studies with the glnA Delta mutant confirm that glutamine, and not GS, i s the key effector of nitrogen metabolite repression. Additionally, ammoniu m and its immediate product glutamate may also act directly to signal nitro gen sufficiency.