The fission yeast CENP-B protein Abp1 prevents pervasive transcription of repetitive DNA elements

A fission yeast CENP-B homolog Abp1 prevents RNAi-mediated heterochromatin formation at ribosomal DNA repeats

In response to nutritional starvation, living cells sensitively regulate the production rates of molecules required for survival. Under glucose starvation, a facultative heterochromatinization of ribosomal DNA is considered to regulate ribosomal RNA production. However, the molecular mechanism is still unclear. Here, we report a novel function of CENP-B homolog Abp1 in forming facultative heterochromatin at ribosomal DNA repeats. We find that the loss of Abp1 induces an ectopic nucleosome assembly at rDNA repeats. Interestingly, the loss of Abp1 induces two mutually exclusive changes at ribosomal DNA repeats: an excess accumulation of methylation of histone H3 at lysine 9, a hallmark of heterochromatin, and an active RNA polymerase II transcription. This excess heterochromatin represses ribosomal RNA expression and requires RNA interference machinery for its formation. Furthermore, we show that the excess heterochromatin does not affect cellular viability under glucose starvation but prevents the return to the proliferation cycle in recovering glucose-rich conditions. Since glucose starvation rapidly induces partial Abp1 disassociation from ribosomal DNA repeats, we propose that Abp1 regulates activity of RNA polymerase II transcription that is paradoxically required for RNA interference-mediated heterochromatin formation and controls an appropriate level of heterochromatinization at ribosomal DNA repeats under glucose starvation.

The wtf meiotic driver gene family has unexpectedly persisted for over 100 million years

Meiotic drivers are selfish elements that bias their own transmission into more than half of the viable progeny produced by a driver+/driver- heterozygote. Meiotic drivers are thought to exist for relatively short evolutionary timespans because a driver gene or gene family is often found in a single species or in a group of very closely related species. Additionally, drivers are generally considered doomed to extinction when they spread to fixation or when suppressors arise. In this study, we examine the evolutionary history of the wtf meiotic drivers first discovered in the fission yeast Schizosaccharomyces pombe. We identify homologous genes in three other fission yeast species, S. octosporus, S. osmophilus, and S. cryophilus, which are estimated to have diverged over 100 million years ago from the S. pombe lineage. Synteny evidence supports that wtf genes were present in the common ancestor of these four species. Moreover, the ancestral genes were likely drivers as wtf genes in S. octosporus cause meiotic drive. Our findings indicate that meiotic drive systems can be maintained for long evolutionary timespans.

Genome-wide chromosomal association of Upf1 is linked to Pol II transcription in Schizosaccharomyces pombe

Although the RNA helicase Upf1 has hitherto been examined mostly in relation to its cytoplasmic role in nonsense mediated mRNA decay (NMD), here we report high-throughput ChIP data indicating genome-wide association of Upf1 with active genes in Schizosaccharomyces pombe. This association is RNase sensitive, correlates with Pol II transcription and mRNA expression levels. Changes in Pol II occupancy were detected in a Upf1 deficient (upf1Δ) strain, prevalently at genes showing a high Upf1 relative to Pol II association in wild-type. Additionally, an increased Ser2 Pol II signal was detected at all highly transcribed genes examined by ChIP-qPCR. Furthermore, upf1Δ cells are hypersensitive to the transcription elongation inhibitor 6-azauracil. A significant proportion of the genes associated with Upf1 in wild-type conditions are also mis-regulated in upf1Δ. These data envisage that by operating on the nascent transcript, Upf1 might influence Pol II phosphorylation and transcription.