1.Mechanisms For Centromere Repositioning And Reroductive Barrier In Schizosaccharomyces Pombe 2.Mechanisms For Centromeric Epigenetic Stability And Heterochromatin Distribution In Schizosaccharomyces Pombe

Centromeres dictate the sites for kinetochore assembly on chromosomes,while their own position on each chromosome is determined epigenetically by a specific histone H3 variant CENP-A.For all eukaryotic species,the chromosomal position of each centromere is distinct and inherited with high fidelity,although the mechanisms underlying the epigenetic stability and its functional significance remain largely unknown.Here in the fission yeast Schizosaccharomyces pombe,we show that mutations in inner kinetochore components influence centromeric chromatin organization to various levels.In extreme cases,a single deletion of wip1,mhf1 and mhf2(the conserved CENP-T-W-S-X complex subunits)or double deletions of cnp3(a homologue of mammalian CENP-C)and fta6(a pombe specific component)induce centromere repositioning-inactivation of the original centromere and formation of a neocentromere-in one of the three chromosomes at random.Neocentromeres tend to locate in pericentromeric heterochromatin regions,although heterochromatin is not required for centromere inactivation.Cells carrying a neocentromere are competent in mitosis and in meiosis of homozygotes.However,when these cells are crossed to cells carrying the original centromere,the progeny suffers severe lethality due to defects in meiotic chromosome segregation.These results recapitulate a meiosis barrier that could initiate genetic divergence between two populations with mismatched centromeres,documenting a potential role of the Evolutionary New Centromeres(ENCs)in speciation.Distinct chromatin organization features,such as centromeres and heterochromatin domains,are inherited epigenetically.However,mechanisms that modulate the accuracy of epigenetic inheritance,especially at the individual nucleosome level,are not well understood.Here,we characterize Ccp1,a homolog of histone chaperone Vps75 that functions in centromere chromatin duplication and heterochromatin maintenance in fission yeast.We show that Ccpl is enriched at the central core regions of the centromeres and among all histone chaperones characterized,deletion of ccpl gene uniquely reduces the rate of epigenetic switching manifested as CEN-PEV(position effect variegation).In contrast,gene deletion of other histone chaperones either elevates the PEV switching rates or does not affect centromeric PEV.Ccpl and the kinetochore components(Mis6 and Sim4)are mutually dependent for centromere or kinetochore association at the proper levels.Moreover,Ccp1 influences heterochromatin distribution at multiple loci in the genome,including the subtelomeric and the pericentromeric regions.We also found that Gar2,a protein predominantly enriched in the nucleolus,functions similarly to Ccpl in modulating the epigenetic stability of centromeric regions although its mechanism remains unclear.Together,our results identify Ccp1 as an important player in modulating epigenetic stability and maintaining proper organization of multiple chromatin domains throughout the genome.