THE SWI5 ZINC-FINGER AND GRF10 HOMEODOMAIN PROTEINS BIND DNA COOPERATIVELY AT THE YEAST HO PROMOTER

SWI5 encodes a zinc-finger protein required for expression of the yeas t HO gene. Using Swi5 protein that was purified from a bacterial expre ssion system, we previously isolated a yeast factor that stimulates bi nding of Swi5 to the HO promoter. N-terminal amino acid sequence analy sis identified the Swi5 stimulatory factor as the product of the GRF10 gene, which encodes a yeast homeodomain protein. GRF10, also known as PHO2 and BAS2, is a transcriptional activator of the PHO5 acid phosph atase gene and the HIS4 histidine biosynthesis gene. Grf10 protein pur ified from a bacterial expression system binds DNA cooperatively with Swi5 in vitro. Analysis of dissociation rates indicates that the Grf10 -Swi5-DNA complex has a longer half-life than protein-DNA complexes th at contain only Swi5 or Grf10. Finally, we show that HO expression is reduced in yeast strains containing grf10 null mutations and that full expression of a heterologous promoter containing a SWI5-dependent HO upstream activation sequence element requires GRF10.