CONNECTIONS BETWEEN THE RAS-CYCLIC AMP PATHWAY AND G(1)-CYCLIN EXPRESSION IN THE BUDDING YEAST SACCHAROMYCES-CEREVISIAE

We have identified two processes in the G1 phase of the Saccharomyces cerevisiae cell cycle that are required before nutritionally arrested cells are able to return to proliferative growth. The first process re quires protein synthesis and is associated with increased expression o f the G1 cyclin gene CLN3. This process requires nutrients but is inde pendent of Ras and cyclic AMP (cAMP). The second process requires cAMP . This second process is rapid, is independent of protein synthesis, a nd produces a rapid induction of START-specific transcripts, including CLN1 and CLN2. The ability of a nutritionally arrested cell to respon d to cAMP is dependent on completion of the first process, and this is delayed in cells carrying a CLN3 deletion. Mating pheromone blocks th e cAMP response but does not alter the process upstream of Ras-cAMP. T hese results suggest a model linking the Ras-cAMP pathway with regulat ion of G1 cyclin expression.