Chromo domain proteins in balanced dosage together with boundary elements cooperate in organising the mating-type chromatin in fission yeast

The chromatin in the cell nucleus has a spatial organisation. For example, in the fission yeast, Schizosaccharomyces pombe, transcriptionally repressed heterochromatin is found at the nuclear membrane (NM). The centromeres and the mating-type region localise in the proximity of the spindle pole body (SPB), while the telomeres are found on the opposite side of the nucleus in the proximity of the nucleolus. In a previous study we used the mating-type region as a model to study the driving force behind nuclear organisation. We proposed two mutually exclusive models to explain what determines the localisation of the mating-type region. The first model suggests that solely the amount of heterochromatin in the region affects the localisation, while the other model stipulates that the boundary elements together with heterochromatin formation anchor the mating-type region in the NM in the vicinity of the SPB. Here, we present data that disproves the first model. We found that in a strain expressing tripled amounts of the chromodomain protein Swi6, a structural component of heterochromatin, the mating-type region was delocalised from the proximity of the SPB. A strain deleted of the histone deacetylase clr3 also had a delocalised mating-type locus. Interestingly, a strain with a point-mutation in clr3-735 producing an enzymatically inactive protein in normal amounts showed an intermediate phenotype. Most importantly, we identify the chromodomain proteins, Chp1 and Chp2, as crucial factors for correct subnuclear localisation of the matingtype region. We suggest that boundary elements together with chromodomain proteins in balanced dosage and composition cooperate in organising the mating-type chromatin. Introduction In the cell nucleus the DNA is packed into nucleosomes by wrapping the DNA around an octameric complex of histones. The histones are globular proteins with unstructured N-terminal tails protruding from the nucleosomes. These N-terminal tails are subjected to modifications resulting in two main types of chromatin; active euchromatin