Growth of on ammonia and glutamine decreases the expression of many nitrogen catabolic genes to low levels. To discriminate between ammonia- and glutamine-driven repression of and , a gln1-37 mutant was used. This mutant is not able to convert ammonia into glutamine. Glutamine-limited continuous cultures were used to completely derepress the expression of and Following an ammonia pulse, the expression of and decreased while the intracellular glutamine concentration remained 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 was not decreased by an ammonia pulse but solely by a glutamine pulse. Analysis of the mRNA levels of and showed that the response of the two biosynthetic genes, and , to ammonia and glutamine in the wild-type and gln1-37 was not due to changes in general transcription of biosynthetic genes. Ure2p has been shown to be an essential element for nitrogen-regulated gene expression. Deletion of in the gln1-37 background prevented repression of gene expression by ammonia, showing that the ammonia-induced repression is not caused by a general stress response but represents a specific signal for nitrogen catabolite regulation.


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