Regulation of Glutamine Synthetase from Saccharomyces cerevisiae by Repression, Inactivation and Proteolysis

Abstract

Glutamine synthetase activity is modulated by N repression and by 2 distinct inactivation processes. Addition of glutamine to exponentially grown yeast leads to enzyme inactivation. 50% of glutamine synthetase activity is lost after 30 min (a quarter of the generation time). Removing glutamine from the growth medium results in a rapid recovery of enzyme activity. A regulatory mutation (gdhCR mutation) suppresses this inactivation by glutamine in addition to its depressing effect on enzymes involved in N catabolism. The gdhCR mutation also increases the level of proteinase B in exponentially grown yeast. Inactivation of glutamine synthetase is also observed during N starvation. This inactivation is irreversible and probably consists of a proteolytic degradation. Strains bearing proteinase A, B and C mutations are no longer inactivated under N starvation.