2 MUTUALLY EXCLUSIVE REGULATORY SYSTEMS INHIBIT UAS(GATA), A CLUSTER OF 5'-GAT(A T)A-3' UPSTREAM FROM THE UGA4 GENE OF SACCHAROMYCES-CEREVISIAE/

The S.cerevisiae Uga43(Dal80) protein down-regulates the expression of multiple nitrogen pathway genes. It contains a zinc-finger motif simi lar to the DNA-binding domain of the vertebrate GATA family of transcr iption factors; this domain is known to direct binding to 5'-GATA-3' c ore sequences, The inducible UGA4 gene, which encodes the specific gam ma-aminobutyrate permease, undergoes strong repression by Uga43p, This study shows that the 5' region of UGA4 contains a UAS element made of four directly repeated 5'-CGAT(A/T) AG-3' sequences, This element, ca lled UAS(GATA), can potentially confer to the UGA4 gene high-level exp ression in the absence of inducer, but this potential activity is inhi bited by two distinct repression systems. One system is Uga43p-depende nt; it operates in cells grown on a poor nitrogen source, The other is the nitrogen repression system, which relies on Ure2p and glutamine a nd operates when a good nitrogen source is present, Nitrogen repressio n also blocks the synthesis of Uga43p, making the two repression syste ms mutually exclusive, Previous studies have shown that expression sup ported by 5'-GATA-3'-containing UAS elements requires Gln3p, another g lobal nitrogen regulatory factor containing a GATA zinc-finger domain. Although Gln3p contributes to UAS(GATA) activity, evidence suggests t hat a second factor can potentially direct expression through UAS(GATA ) Expression conferred by this putative factor is subject to both Uga4 3p- and Ure2p-mediated repression, The role of UAS(GATA) in the expres sion of the UGA4 gene is discussed in relation to its sensitivity to t he two distinct repression systems.