Abstract
In Saccharomyces cerevisiae commitment to cell division occurs at a point in G1 termed Start. This important transition is regulated by the cyclin-dependent kinase Cdc28, in association with the G1 cyclins Cln1, 2 and 3. Transcription of the G1 cyclins is induced by the transcription factor complexes SBF (Swi4-Swi6) and MBF (Mbp1-Swi6); however, data suggest that other proteins are also able to regulate their expression. We previously identified Rme1, a transcription factor with a well documented role in negatively regulating IME1 expression and meiosis, as an activator of CLN2 transcription. We now show that Rme1 acts through two specific Rme1 response elements in the CLN2 promoter to induce expression of the gene. We have analysed in detail the timing of RME1 transcription at the end of mitosis and in G1, and the roles of the transcription factors Ace2 and Swi5 in mediating this expression. We also demonstrate that the Rme1 protein is cell cycle regulated, peaking in G1 and appearing in the nucleus at this time. Finally, the role of RME1 in cell cycle regulation is confirmed by the observation of periodic RME1 expression in diploid cells, where it has no IME1 repressor function; this finding emphasises its role in the regulation of CLN2 expression in G1.