Supplementary Legends from <i>mmi1</i> and <i>rep2</i> mRNAs are novel RNA targets of the Mei2 RNA binding protein during early meiosis in <i>Schizosaccharomyces pombe</i>

The RNA-binding protein Mei2 is crucial for meiosis in <i>Schizosaccharomyces pombe.</i> In <i>mei2</i> mutants, pre-meiotic S-phase is blocked, along with meiosis. Mei2 binds a long non-coding RNA (lncRNA) called meiRNA, which is a ‘sponge RNA’ for the meiotic inhibitor protein Mmi1. The interaction between Mei2, meiRNA and Mmi1 protein is essential for meiosis. But <i>mei2</i> mutants have stronger and different phenotypes than meiRNA mutants, since <i>mei2Δ</i> arrests before pre-meiotic S, while the meiRNA mutant arrests after pre-meiotic S but before meiosis. This suggests Mei2 may bind additional RNAs. To identify novel RNA targets of Mei2, which might explain how Mei2 regulates pre-meiotic S, we used RNA Immunoprecipitation and Cross-linking Immunoprecipitation. In addition to meiRNA, we found the mRNAs for <i>mmi1</i> (which encodes Mmi1) and for the S-phase transcription factor <i>rep2</i>. There were also three other RNAs of uncertain relevance. We suggest that at meiotic initiation, Mei2 may sequester <i>rep2</i> mRNA to help allow pre-meiotic S, and then may bind both meiRNA and <i>mmi1</i> mRNA to inactivate Mmi1 at two levels, the protein level (as previously known), and also the mRNA level, allowing meiosis. We call Mei2–meiRNA a ‘double sponge’ (i.e. binding both an mRNA, and its encoded protein).