Mbf1 ensures Polycomb silencing by protecting mRNA from degradation by Pacman.

Under stress conditions, the coactivator Multiprotein bridging factor 1 (Mbf1) translocates from the cytoplasm into the nucleus to induce stress-response genes. However, its role in the cytoplasm, where it is mainly located, has remained elusive. Here, we show that Mbf1 associates with mRNA and protects it from degradation by the exoribonuclease Pacman (Pcm), thereby ensuring Polycomb silencing. In genetic studies, loss of function enhanced a Polycomb phenotype in Polycomb group mutants, and was accompanied by a significant reduction in mRNA expression. Furthermore, a mutation suppressed the Polycomb phenotype and restored the expression level of mRNA, while overexpression exhibited the Polycomb phenotype in the mutant but not in the wild-type background. , Mbf1 protected RNA from Pcm activity. Our results suggest that Mbf1 buffers fluctuations in Pcm activity to maintain an mRNA expression level sufficient for Polycomb silencing.