Cell cycle-specific expression of the SWI4 transcription factor is required for the cell cycle regulation of HO transcription.

Expression of the HO endonuclease triggers mating-type switching in Saccharomyces cerevisiae. Transcription of the HO gene is start-dependent and restricted to the late G1/early S phase of haploid mother cells. The HO promoter contains 10 copies of a cell cycle-regulated upstream activation sequence, which is activated by SWI4 and SWI6. SWI4 mRNA levels vary at least 10-fold throughout the cell cycle and rise sharply just before the rise in HO mRNA levels. Constitutive synthesis of SWI4 mRNA leads to constitutive synthesis of HO mRNA. These data suggest that the cell cycle regulation of SWI4 mRNA is required for the tight cell cycle regulation of HO transcription. High-level constitutive synthesis of SWI4 also suppresses swi5 and swi6 mutations, suggesting that SWI4 is the predominant activator of HO transcription and that mutations in negative regulators of SWI4 could be isolated as suppressors of swi6 mutations. One recessive suppressor of swi6 (ssx1-1) that allowed high-level expression of SWI4 during alpha-factor arrest and constitutive expression of both SWI4 and HO after release from the arrest was isolated. This result suggests that SSX1 has a negative regulatory role in the cell-cycle regulation of SWI4 mRNA accumulation.