The Clr4 methyltransferase determines the subnuclear localization of the mating-type region in fission yeast

The genome has a non-random spatial distribution in the cell nucleus. In Schizosaccharomyces pombe, it has been shown that the centromeres, telomeres and the mating-type region localize to the nuclear membrane (NM), the former by attaching to the spindle pole body (SPB). In addition, reporter genes inserted into these areas are transcriptionally repressed because of the formation of specialized chromatin structures. Performing live cell analysis we found that in a wild-type strain the mating-type region was positioned in the proximity of the SPB, the location where the pericentromeric heterochromatin is also found. In a strain lacking the histone methyltransferase Clr4, crucial for the formation of heterochromatin, the mating-type region had a random localization in the nucleus. Moreover, in a strain in which the two boundary elements IR-L and IR-R had been deleted, the mating-type region was displaced from its position at the proximity of the SPB, but remained in the vicinity of the NM. Moreover, in all investigated strains with silencing deficiencies the distance between the mating- type region and the SPB increased. This result indicates a correlation between transcriptional derepression and displacement of the region. Two different models of how the mating- type chromatin is organized in the nucleus are discussed.