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Saccharomyces cerevisiae cells can sense the quality of the nitrogen source in their environment, enabling them to utilize preferred nitrogen-containing compounds over nonpreferred ones or to express pathways for the utilization of alternative nitrogen sources when the preferred ones have been consumed. Although very little is known about the sensing mechanism itself, work over the last decade has led to the discovery of a set of regulatory proteins, the GATA factors, whose role is to regulate, in both positive and negative directions, the expression of pathways of nitrogen assimilation in yeast. These proteins, Gln3p (26), Nil1p/Gat1p (10, 44), Dal80p/Uga43p (12, 13), and Nil2p/Gzf3p/Deh1p (11, 34, 42), are involved in a complex set of regulatory loops, competition for GATA binding sites, and possibly even some autoregulation. Recently, the coactivator Ada1p, isolated as Gan1p, was identified as a link between the GATA binding proteins and the basal transcriptional machinery (41). Global nitrogen repressor Ure2p is believed to interact with Gln3p to obtain appropriate expression of a variety of nitrogen assimilatory pathways (3; reviewed by Magasanik [23]).

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