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A DNA binding protein (RAP1, previously called SBF-E) has been shown to bind to
putative regulatory sites at both yeast mating-type silencers, yet is not the
product of genetically identified regulators of the silent loci. Here, we report
the purification of RAP1 by DNA affinity chromatography, and the isolation of
its gene from a lambda gt11 genomic library using antibodies raised against the
protein. Disruption of the chromosomal copy of this gene is lethal. We show that
RAP1 protein also binds in vitro to the upstream activation site (UAS) of MAT
alpha and ribosomal protein genes. In addition, we show that two different
UAS-associated RAP1 binding sites can substitute in vivo for a silencer binding
site. Our results suggest that RAP1 may be a transcriptional regulator that can
play a role in either repression or activation of transcription, depending upon
the context of its binding site.
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