Fission yeast mutants that alleviate transcriptional silencing in centromeric flanking repeats and disrupt chromosome segregation

In the fission yeast Schizosaccharomyces pombe genes are transcriptionally silenced when placed within centromeres, within or close to the silent mati ng-type loci or adjacent to telomeres. Factors required to maintain mating- type silencing also affect centromeric silencing and chromosome segregation . We isolated mutations that alleviate repression of market genes in the in verted repeats flanking the central core of centromere I. Mutations csp1 to 13 (centromere: suppressor of position effect) defined 12 loci. Tell of th e csp mutants have no effect on mat2/3 or telomere silencing. All csp mutan ts allow some expression of genes in the centromeric flanking repeat, but e xpression in the central core is undetectable. Consistent with defective ce ntromere structure and function, chromosome loss rates are elevated in all csp mutants. Mutants csp1 to 6 are temperature-sensitive lethal and csp3 an d csp6 cells are defective in mitosis at 36 degrees. csp7 to 13 display a h igh incidence of lagging chromosomes oil late anaphase spindles. Thus, by s creening for mutations that disrupt silencing in the flanking region of a f ission yeast centromere a novel collection of mutants affecting centromere architecture and chromosome segregation has been isolated.