ANALYSIS OF THE SCHIZOSACCHAROMYCES-POMBE CYCLIN PUC1 - EVIDENCE FOR A ROLE IN CELL-CYCLE EXIT

The puc1+ gene, encoding a G1-type cyclin from the fission yeast Schiz osaccharomyces pombe, was originally isolated by complementation in th e budding yeast Saccharomyces cerevisiae. Here, we report the molecula r characterization of this gene and analyse its role in S. pombe. We f ail to identify any function of this cyclin at the mitotic G1/S transi tion in S. pombe, but demonstrate that it does function in exit from t he mitotic cycle. Expression of the puc1+ gene is increased during nit rogen starvation, and puc1+ affects the timing of sexual development i n response to starvation. Overexpression of the puc1 protein blocks se xual development, and rescues pat1ts cells, which would otherwise unde rgo a lethal meiosis. We conclude that puc1 contributes to negative re gulation of the timing of sexual development in fission yeast, and fun ctions at the transition between cycling and non-cycling cells.