Transcriptional silencing is known to occur at centromeres, telomeres and the 
mating type region in the nucleus of fission yeast, Schizosaccharomyces pombe. 
Mating-type silencing factors have previously been shown also to affect 
transcriptional repression within centromeres and to some extent at telomeres. 
Mutations in the clr4+, rik1+ and swi6+ genes dramatically reduce silencing at 
certain centromeric regions and cause elevated chromosome loss rates. Recently, 
Swi6p was found to co-localise with the three silent chromosomal regions. Here 
the involvement of clr4+, rik1+ and swi6+ in centromere function is investigated 
in further detail. Fluorescence in situ hybridisation (FISH) was used to show 
that, as in swi6 mutant cells, centromeres lag on late anaphase spindles in clr4 
and rik1 mutant cells. This phenotype is consistent with a role for these three 
gene products in fission yeast centromere function. The Swi6 protein was found 
to be delocalised from all three silent chromosomal regions, and dispersed 
within the nucleus, in both clr4 and rik1 mutant cells. The phenotypic 
similarity observed in all three mutants is consistent with the products of both 
the clr4+ and rik1+ genes being required to recruit Swi6p to the centromere and 
other silent regions. Mutations in clr4, rik1 and swi6 also result in elevated 
sensitivity to reagents which destabilise microtubules and show a synergistic 
interaction with a mutation in the beta-tubulin gene (nda3). These observations 
suggest that clr4+ and rik1+ must play a role in the assembly of Swi6p into a 
transcriptionally silent, inaccessible chromatin structure at fission yeast 
centromeres which is required to facilitate interactions with spindle 
microtubules and to ensure normal chromosome segregation.