CDC6 IS AN UNSTABLE PROTEIN WHOSE DE-NOVO SYNTHESIS IN G(1) IS IMPORTANT FOR THE ONSET OF S-PHASE AND FOR PREVENTING A REDUCTIONAL ANAPHASE IN THE BUDDING YEAST SACCHAROMYCES-CEREVISIAE

S phase entry depends on cyclin-dependent kinases whose activation during late G(1) is due partly to the synthesis of unstable cyclin subunits. We identify here a second type of unstable protein, Cdc6, whose synthesis during G(1) is important for initiation of DNA replication. The CDC6 gene is normally transcribed at the end of mitosis, but in cells with a prolonged G(1) phase there is a second burst of transcription in late G(1). The former is due to Swi5, while the latter is due to MBF or SBF transcription factors. Small G(1) cells that cannot synthesize Cdc6 in late G(1) progress through 8 phase very slowly. Cells that transcribe CDC6 neither at the end of mitosis nor in late G(1) fail to replicate DNA but, despite this, undergo mitosis and produce daughter cells with fractional DNA contents. This 'reductional' anaphase occurs with almost wild-type kinetics and depends on the activity of G(2) cyclins. Thus, cells that fail to duplicate chromosomes due to a cdc6 defect cannot prevent the onset of mitosis, unlike other mutants with replication defects. We show, by fluorescence in situ hybridization, that chromosomes which remain unduplicated due to a lack of Cdc6 synthesis are segregated intact to spindle poles during the 'reductional' anaphase.