The role of ammonia metabolism in nitrogen catabolite repression in Saccharomyces cerevisiae

Saccharomyces cerevisiae is able to use a wide variety of nitrogen sources for growth. Not all nitrogen sources support growth equally well. In order to select the best out of a large diversity of available nitrogen sources, the yeast has developed molecular mechanisms. These mechanisms consist of a sensing mechanism and a regulatory mechanism which includes induction of n eeded systems, and repression of systems that are not beneficial. The first step in use of most nitrogen sources is its uptake via more or less specif ic permeases. Hence the first level of regulation is encountered at this le vel. The next step is the degradation of the nitrogen source to useful buil ding blocks via the nitrogen metabolic pathways. These pathways can be divi ded into routes that lead to the degradation of the nitrogen source to ammo nia and glutamate, and routes that lead to the synthesis of nitrogen contai ning compounds in which glutamate and glutamine are used as nitrogen donor. Glutamine is synthesized out of ammonia and glutamate. The expression of t he specific degradation routes is also regulated depending on the availabil ity of a particular nitrogen source. Ammonia plays a central role as interm ediate between degradative and biosynthetic pathways. It not only functions as a metabolite in metabolic reactions but is also involved in regulation of metabolic pathways at several levels. This review describes the central role of ammonia in nitrogen metabolism This role is illustrated at the leve l of enzyme activity, translation and transcription. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.