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The UAS of the yeast gene encoding the glycolytic enzyme phosphoglycerate kinase (PGK) contains several different sequence elements involved in transcriptional activation. These elements include the activator core sequence, which is bound by the RAP1 protein, and three copies of the pentamer sequence 5' CTTCC 3'. Upstream of the activator core sequence is a region (Yfp), identified as the site of a strong DNA-protein interaction. The Yfp region contains the consensus binding site for the factor ABF1. We have purified the Y protein, which binds to the Yfp region, to homogeneity. The Y protein migrates as a doublet on SDS-polyacrylamide gel electrophoresis with an apparent molecular weight of 125 KDa. These properties are similar to those of ABF1. ABF1 synthesised in vitro bound strongly to the Yfp region and formed a gel retardation complex of identical mobility to the complex formed by the Y protein. UAS1 of the pyruvate kinase gene (PYK1) promoter contains a RAP1 binding site and single copy of the CTTCC sequence. We have now identified an ABF1 binding site close to the RAP1 binding site and CTTCC sequence in the PYK1 promoter. This site is strongly bound by ABF1 in vitro. The organisation of the PGK and PYK1 UASs is thus similar to each other and to the transcriptional silencer HMR(E) which also contains these sequences.