Histone acetylation may act to mark and maintain transcriptionally active or inactive chromosomal domains through the cell cycle and in different lineages. A novel role for histone acetylation in centromere regulation has been identified. Exposure of fission yeast cells to TSA, a specific inhibitor of histone deacetylase, interferes with repression of marker genes in centromeric heterochromatin, causes chromosome loss, and disrupts the localization of Swi6p, a component of centromeric heterochromatin. Transient TSA treatment induces a heritable hyperacetylated state in centromeric chromatin that is propagated in lineages in the absence of drug. This state is linked in cis to the treated centromere locus and correlates with inheritance of functionally defective centromeres and persistent chromosome segregation problems. Thus, assembly of fully functional centromeres is partly imprinted in the underacetylated or transcriptionally silent state of centromeric chromatin.

We have studied telomere length in Schizosaccharomyces pombe strains carrying mutations affecting cell cycle checkpoints, DNA repair, and regulation of the Cdc2 protein kinase. Telomere shortening was found in rad1, rad3, rad17, and rad26 mutants. Telomere lengths in previously characterized rad1 mutants paralleled the replication checkpoint proficiency of those mutants. In contrast, rad9, chk1, hus1, and cds1 mutants had intact telomeres. No difference in telomere length was seen in mutants affected in the regulation of Cdc2, whereas some of the DNA repair mutants examined had slightly longer telomeres than did the wild type. Overexpression of the rad1 ϩ gene caused telomeres to elongate slightly. The kinetics of telomere shortening was monitored by following telomere length after disruption of the rad1 ϩ gene; the rate was ϳ1 nucleotide per generation. Wild-type telomere length could be restored by reintroduction of the wildtype rad1 ϩ gene. Expression of the Saccharomyces cerevisiae RCK1 protein kinase gene, which suppresses the radiation and hydroxyurea sensitivity of Sz. pombe checkpoint mutants, was able to attenuate telomere shortening in rad1 mutant cells and to increase telomere length in a wild-type background. The functional effects of telomere shortening in rad1 mutants were assayed by measuring loss of a linear and a circular minichromosome. A minor increase in loss rate was seen with the linear minichromosome, and an even smaller difference compared with wild-type was detected with the circular plasmid.

hda1+ (histone deacetylase 1) is a fission yeast gene which is highly similar in sequence to known histone deacetylase genes in humans and budding yeast. We have investigated if this putative histone deacetylase contributes to transcriptional silencing in the fission yeast Schizosaccharomyces pombe. A precise deletion allele of the hda1+ open reading frame was created. Cells lacking the hda1+ gene are viable. However, genetic analysis reveals that cells without hda1 + display enhanced gene repression/silencing of marker genes, residing adjacent to telomeres, close to the silent mating-type loci and within centromere I. This phenotype is very similar to that recently reported for rpd3 mutants both in Drosophila and budding yeast. No defects in chromosome segregation or changes in telomere length were detected. Cells lacking the hda1+ gene display reduced sporulation. Growth of hda1 cells is partially inhibited by low concentrations of Trichostatin A (TSA), a known inhibitor of histone deacetylase enzymes. TSA treatment is also able to overcome the enhanced silencing found in heterochromatic regions of hda1 cells. These results indicate a genetic redundancy with respect to deacetylase genes and partially overlapping functions of these in fission yeast. The significance of these results is discussed in the light of recent discoveries from other eukaryotes.

We have investigated the effects of inhibition of histone de-acetylase activity on silencing at the silent mating-type loci in fission yeast. Treatment of exponentially growing cells with the histone deacetylase inhibitor, trichostatin A (TSA), resulted in derepression of a marker gene inserted 150 bp distal from the silent mat3-M locus. The natural targets for the silencing mechanism in this region were only partially derepressed and the activation appeared to be asymmetric, i.e. the mat2-P cassette remained silent at concentrations that clearly partially derepressed the mat3-M cassette. We further noted that treatment of wild-type h90 cells resulted in the generation of altered sporulation phenotypes, indicating that the treatment affected the expression of mating-type genes and/or mating-type switching. The results are discussed in the light of recent accumulated data regarding the role of deacetylation for silencing in other species.

Transcriptional silencing can reflect heritable, epigenetic inactivation of genes, either singly or in groups, during the life-time of an organism. This phenomenon is exemplified by parent-of-origin-specific inactivation events (genomic imprinting) for a subset of mammalian autosomal genes, such as H19. Very little is known, however, about the timing and mechanism(s) of silencing of the paternal H19 allele during mouse development. Using a novel in situ approach, we present evidence that the silencing of the paternal H19 allele is progressive in the trophectodermal lineage during early mouse development and generates variegated expression patterns. The silencing process apparently involves recruitment of histone deacetylases since the mosaic paternal-specific H19 expression reappears in trichostatin A-treated mouse conceptuses, undergoing in vitro organogenesis. Moreover, the paternal H19 alleles of PatDup.d7 placentas, in which a region encompassing the H19 locus of chromosome 7 is bipaternally derived, partially escape the silencing process and are expressed in a variegated manner. We suggest that allele-specific silencing of H19 share some common features with chromatin-mediated silencing in position-effect variegation.