The fission yeast APC activator Ste9 is regulated by mRNA decay

Fission yeast Ste9/ Srw1 is a family member of the Fizzy- related APC activators that promote the ubiquitination and degradation of mitotic cyclins and other substrates at the end of mitosis and G1. These proteins are highly regulated during the cell cycle at the level of gene transcription and protein phosphorylation in order to guarantee the correct order of events during the cell cycle. Here we propose mRNA decay as a novel mechanism that regulates ste9+ gene expression during the cell cycle. We have characterized the elements in the 3'UTR of the ste9 mRNA responsible for this mechanism. Moreover, we demonstrate that the instability of ste9 mRNA is important for downregulating Ste9 levels in G(2), allowing appropriate cyclin B accumulation to promote timely entry into mitosis.