1. Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 94303, USA

Correspondence to: Steven I. Reed Correspondence and requests for materials should be addressed to S.I.R. (Email: e-mail: sreed@scripps.edu).

Abstract

In yeast and somatic cells, mechanisms ensure cell-cycle events are initiated only when preceding events have been completed¹. In contrast, interruption of specific cell-cycle processes in early embryonic cells of many organisms does not affect the timing of subsequent events², indicating that cell-cycle events are triggered by a free-running cell-cycle oscillator. Here we present evidence for an independent cell-cycle oscillator in the budding yeast Saccharomyces cerevisiae. We observed periodic activation of events normally restricted to the G1 phase of the cell cycle, in cells lacking mitotic cyclin-dependent kinase activities that are essential for cell-cycle progression. As in embryonic cells, G1 events cycled on schedule, in the absence of S phase or mitosis, with a period similar to the cell-cycle time of wild-type cells. Oscillations of similar periodicity were observed in cells responding to mating pheromone in the absence of G1 cyclin (Cln)- and mitotic cyclin (Clb)-associated kinase activity, indicating that the oscillator may function independently of cyclin-dependent kinase dynamics. We also show that Clb-associated kinase activity is essential for ensuring dependencies by preventing the initiation of new G1 events when cell-cycle progression is delayed.