Suppression of Meiotic Recombination by CENP-B Homologs in Schizosaccharomyces pombe

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.

Human artificial chromosome: Chromatin assembly mechanisms and CENP-B

The centromere is a specialized chromosomal locus required for accurate chromosome segregation. Heterochromatin also assembles around centromere chromatin and forms a base that supports sister chromatid cohesion until anaphase begins. Both centromere chromatin and heterochromatin assemble on a centromeric DNA sequence, a highly repetitive sequence called alphoid DNA (α-satellite DNA) in humans. Alphoid DNA can form a de novo centromere and subsequent human artificial chromosome (HAC) when introduced into the human culture cells HT1080. HAC is maintained stably as a single chromosome independent of other human chromosomes. For de novo centromere assembly and HAC formation, the centromere protein CENP-B and its binding sites, CENP-B boxes, are required in the repeating units of alphoid DNA. CENP-B has multiple roles in de novo centromere chromatin assembly and stabilization and in heterochromatin formation upon alphoid DNA introduction into the cells. Here we review recent progress in human artificial chromosome construction and centromere/heterochromatin assembly and maintenance, focusing on the involvement of human centromere DNA and CENP-B protein.

Transposable Element Domestication As an Adaptation to Evolutionary Conflicts

Transposable elements (TEs) are selfish genetic units that typically encode proteins that enable their proliferation in the genome and spread across individual hosts. Here we review a growing number of studies that suggest that TE proteins have often been co-opted or 'domesticated' by their host as adaptations to a variety of evolutionary conflicts. In particular, TE-derived proteins have been recurrently repurposed as part of defense systems that protect prokaryotes and eukaryotes against the proliferation of infectious or invasive agents, including viruses and TEs themselves. We argue that the domestication of TE proteins may often be the only evolutionary path toward the mitigation of the cost incurred by their own selfish activities.

Centromere Stability: The Replication Connection

The fission yeast centromere, which is similar to metazoan centromeres, contains highly repetitive pericentromere sequences that are assembled into heterochromatin. This is required for the recruitment of cohesin and proper chromosome segregation. Surprisingly, the pericentromere replicates early in the S phase. Loss of heterochromatin causes this domain to become very sensitive to replication fork defects, leading to gross chromosome rearrangements. This review examines the interplay between components of DNA replication, heterochromatin assembly, and cohesin dynamics that ensures maintenance of genome stability and proper chromosome segregation.

Multiple Transcriptional and Post-transcriptional Pathways Collaborate to Control Sense and Antisense RNAs of Tf2 Retroelements in Fission Yeast

Retrotransposons are mobile genetic elements that colonize eukaryotic genomes by replicating through an RNA intermediate. As retrotransposons can move within the host genome, defense mechanisms have evolved to repress their potential mutagenic activities. In the fission yeast Schizosaccharomyces pombe, the mRNA of Tf2 long terminal repeat retrotransposons is targeted for degradation by the 3'-5' exonucleolytic activity of the exosome-associated protein Rrp6. Here, we show that the nuclear poly(A)-binding protein Pab2 functions with Rrp6 to negatively control Tf2 mRNA accumulation. Furthermore, we found that Pab2/Rrp6-dependent RNA elimination functions redundantly to the transcriptional silencing mediated by the CENP-B homolog, Abp1, in the suppression of antisense Tf2 RNA accumulation. Interestingly, the absence of Pab2 attenuated the derepression of Tf2 transcription and the increased frequency of Tf2 mobilization caused by the deletion of abp1 Our data also reveal that the expression of antisense Tf2 transcripts is developmentally regulated and correlates with decreased levels of Tf2 mRNA. Our findings suggest that transcriptional and post-transcriptional pathways cooperate to control sense and antisense RNAs expressed from Tf2 retroelements.