Promoter-driven splicing regulation in fission yeast

The meiotic cell cycle is modified from the mitotic cell cycle by having a pre- meiotic S phase that leads to high levels of recombination, two rounds of nuclear division with no intervening DNA synthesis and a reductional pattern of chromosome segregation. Rem1 is a cyclin that is only expressed during meiosis in the fission yeast Schizosaccharomyces pombe. Cells in which rem1 has been deleted show decreased intragenic meiotic recombination and a delay at the onset of meiosis I ( ref. 1). When ectopically expressed in mitotically growing cells, Rem1 induces a G1 arrest followed by severe mitotic catastrophes. Here we show that rem1 expression is regulated at the level of both transcription and splicing, encoding two proteins with different functions depending on the intron retention. We have determined that the regulation of rem1 splicing is not dependent on any transcribed region of the gene. Furthermore, when the rem1 promoter is fused to other intron-containing genes, the chimaeras show a meiotic- specific regulation of splicing, exactly the same as endogenous rem1. This regulation is dependent on two transcription factors of the forkhead family, Mei4 ( ref. 2) and Fkh2 ( ref. 3). Whereas Mei4 induces both transcription and splicing of rem1, Fkh2 is responsible for the intron retention of the transcript during vegetative growth and the pre- meiotic S phase.