ACE2 Is Required for Daughter Cell-Specific G1Delay in Saccharomyces cerevisiae

Saccharomyces cerevisiae cells reproduce by budding to yield a mother cell and a smaller daughter cell. Although both mother and daughter begin G1simultaneously, the mother cell progresses through G1more rapidly. Daughter cell G1delay has long been thought to be due to a requirement for attaining a certain critical cell size before passing the commitment point in the cell cycle known as START. We present an alternative model in which the daughter cell-specific Ace2 transcription factor delays G1in daughter cells. Deletion of ACE2 produces daughter cells that proceed through G1at the same rate as mother cells, whereas a mutant Ace2 protein that is not restricted to daughter cells delays G1equally in both mothers and daughters. The differential in G1length between mothers and daughters requires the Cln3 G1cyclin, and CLN3-GFP reporter expression is reduced in daughters in an ACE2-dependent manner. Specific daughter delay elements in the CLN3 promoter are required for normal daughter G1delay, and these elements bind to an unidentified 127-kDa protein. This DNA-binding activity is enhanced by deletion of ACE2. These results support a model in which daughter cell G1delay is determined not by cell size but by an intrinsic property of the daughter cell generated by asymmetric cell division.