REPRESSION OF NITROGEN CATABOLIC GENES BY AMMONIA AND GLUTAMINE IN NITROGEN-LIMITED CONTINUOUS CULTURES OF SACCHAROMYCES-CEREVISIAE

Growth of Saccharomyces cerevisiae on ammonia and glutamine decreases the expression or many nitrogen catabolic genes to low levels. To disc riminate between ammonia-and glutamine-driven repression of GAP1, PUT4 , GDH1 and GLN1, a gln 1-37 mutant was used, This mutant is not able t o convert ammonia into glutamine. Glutamine-limited continuous culture s were used to completely derepress the expression of GAP1, PUT4, GDH1 and CLIVI. Following an ammonia pulse, the expression of GAP1, PUT4 a nd GDH1 decreased while the intracellular glutamine concentration rema ined constant, both in the cytoplasm and in the vacuole. Therefore, it was concluded that ammonia causes gene repression independent of the intracellular glutamine concentration. The expression of GLN1 was not decreased by an ammonia pulse but solely by a glutamine pulse. Analysi s of the mRNA levels of ILV5 and HIS4 showed that the response of the two biosynthetic genes, GDH1 and GLN1, to ammonia and glutamine in the wild-type and gln 1-37 was not due to changes in general transcriptio n of biosynthetic genes. Ure2p has been shown to be an essential eleme nt for nitrogen-regulated gene expression. Deletion of URE2 in the gln 1-37 background prevented repression of gene expression by ammonia, s howing that the ammonia-induced repression is not caused by a general stress response but represents a specific signal for nitrogen cataboli te regulation.