Published online 14 December 2004
Nucleic Acids Research, Vol. 32 No. 22 © Oxford University Press 2004; all rights reserved
Repression of the yeast HO gene by the MAT
2 and MATa1 homeodomain proteins
Waksman Institute and Department of Molecular Biology and Biochemistry, 190 Frelinghuysen Road, 1 Department of Physics and Astronomy and 2 BioMaPS Institute, Rutgers University, Piscataway, NJ 08854-8020, USA
* To whom correspondence should be addressed. Tel: +1 732 445 2905; Fax: +1 732 445 5735; Email: vershon{at}waksman.rutgers.edu
Present addresses: Jonathan R. Mathias, University of Wisconsin, Department of Pediatrics, 2715 Medical Sciences Center, 1300 University Avenue, Madison, WI 53706, USA
Sean E. Hanlon, University of North Carolina, Department of Biology, 203 Fordham Hall, Chapel Hill, NC 27599, USA
Received September 2, 2004; Revised November 5, 2004; Accepted November 18, 2004
The HO gene in Saccharomyces cerevisiae is regulated by a large and complex promoter that is similar to promoters in higher order eukaryotes. Within this promoter are 10 potential binding sites for the a1-
2 heterodimer, which represses HO and other haploid-specific genes in diploid yeast cells. We have determined that a1-2 binds to these sites with differing affinity, and that while certain strong-affinity sites are crucial for repression of HO, some of the weak-affinity sites are dispensable. However, these weak-affinity a1-2-binding sites are strongly conserved in related yeast species and have a role in maintaining repression upon the loss of strong-affinity sites. We found that these weak sites are sufficient for a1-2 to partially repress HO and recruit the Tup1-Cyc8 (Tup1-Ssn6) co-repressor complex to the HO promoter. We demonstrate that the Swi5 activator protein is not bound to URS1 in diploid cells, suggesting that recruitment of the Tup1-Cyc8 complex by a1-2 prevents DNA binding by activator proteins resulting in repression of HO.