| Summary: | The fission yeast centromere is packaged as transcriptionally silent heterochromatin which serves as a platform for kinetochore assembly. The centromere consists of two distinct domains; the outer repeats and the central core. It has been shown previously that these regions associate with distinct sets of proteins, for example, the fission yeast homologue of CENP-A, Cnp1p, is present at the central core, together with Mis6 and Mis12, whilst the heterochromatin protein Swi6 associates with the outer repeats. Marker genes placed in the centromere are transcriptionally silenced. This feature of the fission yeast centromere was utilised to screen for potential kinetochore components or regulators. Mutants with alleviated silencing at the central core were isolated and seven complementation groups identified; sim1 to 7, for silencing in the middle of the centromere. All the sim mutants display chromosome segregation defects and elevated rates of loss of a non-essential minichromosome. This study describes the ongoing characterisation of <i>sims</i> <i>1,6</i> and <i>7</i>. GFP tagged Sim1 associates with the central core of the centromere suggesting that Sim1 is also a novel kinetochore protein. Our working model is that Sim1 may be required for the assembly of Cnp1p chromatin. The <i>sim6</i> mutant is unusual as it alleviates silencing at both the central core and outer repeat regions. In the <i>sim7</i> mutant at the restrictive temperature, Cnp1, a crucial component of the centromere, shows greatly reduced localisation.
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