Abstract
Genetic and molecular studies have indicated that an epigenetic imprint at mat1, the sexual locus of fission yeast, initiates mating type switching. The polar DNA replication of mat1 generates an imprint on the Watson strand. The process by which the imprint is formed and maintained through the cell cycle remains unclear. To understand better the mechanism of imprint formation and stability, we characterized the recruitment of early players of mating type switching at the mat1 region. We found that the switch activating protein 1 (Sap1) is preferentially recruited inside the mat1M allele on a sequence (SS13) that enhances the imprint. The lysine specific demethylases, Lsd1/2, that control the replication fork pause at MPS1 and the formation of the imprint are specifically drafted inside of mat1, regardless of the allele. The CENP-B homolog, Abp1, is highly enriched next to mat1 but it is not required in the process. Additionally, we established the computational signature of the imprint. Using this signature, we show that both sides of the imprinted molecule are bound by Lsd1/2 and Sap1, suggesting a nucleoprotein protective structure defined as imprintosome.
Highlights
Haploid Schizosaccharomyces pombe cells exist as two mating types (MTs), P and M, that switch during cell divisions
Chromatin Immunoprecipitation (ChIP)-sequencing mapping of Abp1, Lsd1/2 and switch activating protein 1 (Sap1) to the h90 mating type region It has been previously reported that Abp1, Lsd1/2 and Sap1 bind to the mat1 region [27, 35, 44]
Several limitations arose from those studies: The strains that were initially used to map Abp1 and Sap1 by ChIP-seq were rearranged in the MT region (h-S), and the length of the reads (30 nucleotides) prevented their unambiguous assignment to mat1, 2 or 3 cassettes [35]
Summary
Haploid Schizosaccharomyces pombe cells exist as two mating types (MTs), P (for plus) and M (for minus), that switch during cell divisions. The mat allele can be replaced efficiently by genetic information contained in one of the two silent donor cassettes mat2P and mat3M [2,3,4]. The H1 homology box (59 bp) is located on the centromere distal (right) side of the cassettes and the H2 homology box (135 bp) on the centromere proximal (left) side [2]. Both sequences are thought to be essential for base pairing during the initiation and resolution steps of the gene conversion process required for MT switching [11, 12]. The H3 box (57 bp) is located to the left of H2 at mat and mat3 [2], and is not required for MT switching [13]
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