Pleiotropic Deficiency in Nitrogen-Uptake Systems and Derepression of Nitrogen-Catabolic Enzymes in npr-1 Mutants of Saccharomyces cerevisiae

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Abstract
The npr-1 mutation has 2 types of effects in S. cerevisiae. Several N-catabolic enzymes are derepressed in the presence of ammonia, glutamine or asparagine, which provoke their repression in a wild-type strain. The activity of several ammonia-sensitive permeases is decreased (from 50 to 100% depending on the permease considered) in npr-1 cells, independently of the N source used for growth. These results favor the idea that the primary effect of the npr-1 mutation is on the permeases and that the derepression of the enzymes is a consequence of the reduced uptake rate of the repressing N compounds. Hence, the product of the npr-1 gene appears to be directly involved in the development of the activity of a set of permeases which transport N compounds and which are regulated by N effectors.