Project description:Imbalances of gene expression in aneuploids containing an abnormal number of chromosomes cause a variety of growth and developmental defects. Aneuploid cells of the fission yeast Schizosaccharomyces pombe are inviable, or very unstable, during mitotic growth. However, S. pombe haploid cells bearing a small portion (less than 20%) of the third chromosome can grow stably as a partial aneuploid. We compared the gene expression profiles of partial aneuploid strains with those of the normal haploid strain using DNA microarray analysis. The expression of genes in disomic or trisomic cells was found to increase approximately in proportion to their copy number. We also found that some genes in the monosomic regions of partial aneuploid strains changed their expression level despite there being no change in copy number. We initially speculated that this change in gene expression was a secondary effect resulting from increased expression of the genes in the disomic or trisomic regions. However, even in an aneuploid strain that bears a minichromosome containing no protein coding genes, genes located within about 50kb of the telomere showed similar increases in expression. This result indicates that the change in expression of the genes in the telomere-adjacent region is not a secondary effect of the increased gene dosage. Examining the distribution of the heterochromoatin protein Swi6 using DNA microarray analysis, we found that binding of Swi6 within ~50kb from the telomere occurred less in partial aneuploid strains compared to euploid strains. These results suggest that additional chromosomes in aneuploid strains could lead to imbalances in gene expression through derepression of heterochromatin. Keywords: Type of experiment: Comparing between partial aneuploid strains and normal haploid strains.
Project description:Proteins of the conserved HP1 family are elementary components of heterochromatin and are generally assumed to play a central role in creating a rigid heterochromatic network that is densely packed and inaccessible to the transcription machinery. In this study we demonstrate that the fission yeast HP1 protein Swi6 exists as a single highly dynamic population and rapidly exchanges in cis and in trans between different heterochromatic regions. Binding to methylated H3K9 or to heterochromatic RNA decelerates Swi6 mobility. We further show that Swi6 is largely dispensable to maintain heterochromatin domains. In contrast, our results disclose an unexpected role of Swi6 in demarcating constitutive heterochromatin from neighboring euchromatin. Our results are consistent with a stochastic model of heterochromatin and imply that heterochromatin is permissive for transcription throughout the cell cycle. Rather than promoting maintenance and spreading of heterochromatin, Swi6 appears to limit these processes to ensure that heterochromatin is appropriately confined.
