Functional organisation of the cell nucleus in the fission yeast, Schizosaccharomyces pombe
Abstract: In eukaryotes the genome adopts a non-random spatial organisation, which is important for gene regulation. However, very little is known about the driving forces behind nuclear organisation. In the simple model eukaryote fission yeast, Schizosaccharomyces pombe, it has been known for a long time that transcriptionally repressed heterochromatin localise to the nuclear membrane (NM); the centromeres attaches to spindle pole body (SPB), while the telomeres are positioned at the NM on the opposite side of the nucleus compared to the SPB. Studies presented in this thesis aimed at advancing our knowledge of nuclear organisation in Schizosaccharomyces pombe.We show that the heterochromatic mating-type region localises to the NM in the vicinity of the SPB. This positioning was completely dependent on Clr4, a histone methyl transferase crucial for the formation of heterochromatin. Additional factors important for localisation were also identified: the chromo domain protein Swi6, and the two boundary elements IR-L and IR-R surrounding this locus. We further identify two other chromo domain proteins; Chp1 and Chp2, as crucial factors for correct subnuclear localisation of this region. From these results we suggest that the boundary elements together with chromodomain proteins in balanced dosage and composition cooperate in organising the mating-type chromatin.Gene regulation can affect the subnuclear localisation of genes. Using nitrogen starvation in S. pombe as a model for gene induction we determined the subnuclear localisation of two gene clusters repressed by nitrogen: Chr1 and Tel1. When repressed these loci localise to the NM, and this positioning is dependent on the histone deacetylase Clr3. During induction the gene clusters moved towards the nuclear interior in a transcription dependent manner.The knowledge gained from work presented in this thesis, regarding nuclear organisation in the S. pombe model system, can hopefully aid to a better understanding of human nuclear organisation.
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