Positive feedback in the activation of Gl cyclins in yeast

Abstract

YEAST cells become committed to the mitotic cell cycle at a stage during Gl called Start1. To enter Start, cells must grow to a critical size. They also require the CDC28 protein kinase and at least one of three Gl-specific cyclins encoded by CLN1, 2, and 3 (refs 2-4). It is thought that Start is triggered by the accumulation of Gl cyclins that bind to the CDC28 kinase and activate it. So what determines the accumulation of Gl cyclins? For CLN1 and CLN2, transcriptional activation could be involved because their RNAs appear transiently during the cell cycle as cells undergo Start5. Here we report that the appearance of CLN1 and CLN2 RNAs depends on an active CDC28 kinase and is stimulated by CLN3 activity. We propose that CDC28 kinase activity due to CLN1 and CLN2 proteins arises through a positive feedback loop which allows CLN proteins to promote their own synthesis. © 1991 Nature Publishing Group.