REGULATING THE HO ENDONUCLEASE IN YEAST

The pedigree of mating-type switching in yeast is determined by the transcription pattern of the HO endonuclease gene, which is expressed during late G1 in mother cells but not at all in daughter cells. The late-G1 specificity of HO transcription depends on a heteromeric factor, SBF, which is composed of the Swi4 and Swi6 proteins. Mother-cell specificity involves a second site-specific DNA-binding factor, Swi5, which is synthesized in the G2 and M phases and only enters the nucleus at the end of mitosis. Swi5 enters mother and daughter nuclei in equal amounts and most is then rapidly degraded. It has been suggested that in mothers but not in daughters some Swi5 protein escapes degradation and persists until SBF is activated in late G1. This subset of Swi5 molecules may constitute a mother cell's memory.