[Show abstract][Hide abstract] ABSTRACT: In Saccharomyces cerevisiae, all H3K4 methylation is performed by a single Set1 Complex (Set1C) that is composed of the catalytic (Set1) and seven other subunits (Swd1, Swd2, Swd3, Bre2, Sdc1, Spp1 and Shg1). It has been known for quite some time that trimethylated H3K4 (H3K4me3) is enriched in the vicinity of meiotic double-strand breaks (DSBs), but the link between H3K4me3 and the meiotic nuclease Spo11 was uncovered only recently. The PHD-containing subunit Spp1, by interacting with H3K4me3 and Mer2, was shown to promote the recruitment of potential meiotic DSB sites to the chromosomal axis allowing their subsequent cleavage by Spo11. Therefore, Spp1 emerged as a key regulator of the H3K4 trimethylation catalyzed by Set1C and of the formation of meiotic DSBs. These findings illustrate the remarkable multifunctionality of Spp1, which not only regulates the catalytic activity of the enzyme (Set1), but also interacts with the deposited mark, and mediates its biological effect (meiotic DSB formation) independently of the complex. As it was previously described for Swd2, and now for Spp1, we anticipate that other Set1C subunits, in addition to regulating H3K4 methylation, may participate in diverse biological functions inside or outside of the complex.
Preview · Article · Mar 2013 · Epigenetics: official journal of the DNA Methylation Society
[Show abstract][Hide abstract] ABSTRACT: The large Mediator (L-Mediator) is a general coactivator of RNA polymerase II transcription and is formed by the reversible association of the small Mediator (S-Mediator) and the kinase-module-harboring Cdk8. It is not known how the kinase module association/dissociation is regulated. We describe the fission yeast Cdk11-L-type cyclin pombe (Lcp1) complex and show that its inactivation alters the global expression profile in a manner very similar to that of mutations of the kinase module. Cdk11 is broadly distributed onto chromatin and phosphorylates the Med27 and Med4 Mediator subunits on conserved residues. The association of the kinase module and the S-Mediator is strongly decreased by the inactivation of either Cdk11 or the mutation of its target residues on the Mediator. These results show that Cdk11-Lcp1 regulates the association of the kinase module and the S-Mediator to form the L-Mediator complex.
[Show abstract][Hide abstract] ABSTRACT: The "CTD code" links the combinatorial potential of the modifications found on the Rpb1 C-terminal domain (CTD) to the growing group of CTD binding effectors. The genetic dissection of serine 2 and serine 7 function within the CTD in both budding and fission yeast reveals distinct in vivo requirement.
[Show abstract][Hide abstract] ABSTRACT: The largest subunit of RNA polymerase II, Rpb1, contains an unusual C-terminal domain (CTD) composed of numerous repeats of the YSPTSPS consensus sequence. This sequence is the target of post-translational modifications such as phosphorylation, glycosylation, methylation and transitions between stereoisomeric states, resulting in a vast combinatorial potential referred to as the CTD code. In order to gain insight into the biological significance of this code, several studies recently reported the genome-wide distribution of some of these modified polymerases and associated factors in either fission yeast (Schizosaccharomyces pombe) or budding yeast (Saccharomyces cerevisiae). The resulting occupancy maps reveal that a general RNA polymerase II transcription complex exists and undergoes uniform transitions from initiation to elongation to termination. Nevertheless, CTD phosphorylation dynamics result in a gene-specific effect on mRNA expression. In this review, we focus on the gene-specific requirement of CTD phosphorylation and discuss in more detail the case of serine 2 phosphorylation (S2P) within the CTD, a modification that is dispensable for general transcription in fission yeast but strongly affects transcription reprogramming and cell differentiation in response to environmental cues. The recent discovery of Cdk12 as a genuine CTD S2 kinase and its requirement for gene-specific expression are discussed in the wider context of metazoa.
Preview · Article · May 2012 · Molecular Microbiology
[Show abstract][Hide abstract] ABSTRACT: Heterochromatin assembly in fission yeast relies on the processing of cognate noncoding RNAs by both the RNA interference
and the exosome degradation pathways. Recent evidence indicates that splicing factors facilitate the cotranscriptional processing
of centromeric transcripts into small interfering RNAs (siRNAs). In contrast, how the exosome contributes to heterochromatin
assembly and whether it also relies upon splicing factors were unknown. We provide here evidence that fission yeast Spf30
is a splicing factor involved in the exosome pathway of heterochromatin silencing. Spf30 and Dis3, the main exosome RNase,
colocalize at centromeric heterochromatin and euchromatic genes. At the centromeres, Dis3 helps recruiting Spf30, whose deficiency
phenocopies the dis3-54 mutant: heterochromatin is impaired, as evidenced by reduced silencing and the accumulation of polyadenylated centromeric
transcripts, but the production of siRNAs appears to be unaffected. Consistent with a direct role, Spf30 binds centromeric
transcripts and locates at the centromeres in an RNA-dependent manner. We propose that Spf30, bound to nascent centromeric
transcripts, perhaps with other splicing factors, assists their processing by the exosome. Splicing factor intercession may
thus be a common feature of gene silencing pathways.
Full-text · Article · Dec 2009 · Molecular and Cellular Biology
[Show abstract][Hide abstract] ABSTRACT: Sister chromatid cohesion is mediated by cohesin, but the process of cohesion establishment during S-phase is still enigmatic. In mammalian cells, cohesin binding to chromatin is dynamic in G1, but becomes stabilized during S-phase. Whether the regulation of cohesin stability is integral to the process of cohesion establishment is unknown. Here, we provide evidence that fission yeast cohesin also displays dynamic behavior. Cohesin association with G1 chromosomes requires continued activity of the cohesin loader Mis4/Ssl3, suggesting that repeated loading cycles maintain cohesin binding. Cohesin instability in G1 depends on wpl1, the fission yeast ortholog of mammalian Wapl, suggestive of a conserved mechanism that controls cohesin stability on chromosomes. wpl1 is nonessential, indicating that a change in wpl1-dependent cohesin dynamics is dispensable for cohesion establishment. Instead, we find that cohesin stability increases at the time of S-phase in a reaction that can be uncoupled from DNA replication. Hence, cohesin stabilization might be a pre-requisite for cohesion establishment rather than its consequence.
[Show abstract][Hide abstract] ABSTRACT: Sister-chromatid cohesion is mediated by cohesin, a ring-shape complex made of four core subunits called Scc1, Scc3, Smc1, and Smc3 in Saccharomyces cerevisiae (Rad21, Psc3, Psm1, and Psm3 in Schizosaccharomyces pombe). How cohesin ensures cohesion is unknown, although its ring shape suggests that it may tether sister DNA strands by encircling them . Cohesion establishment is a two-step process. Cohesin is loaded on chromosomes before replication and cohesion is subsequently established during S phase. In S. cerevisiae, cohesin loading requires a separate complex containing the Scc2 and Scc4 proteins. Cohesin rings fail to associate with chromatin and cohesion can not establish when Scc2 is impaired . The mechanism of loading is unknown, although some data suggest that hydrolysis of ATP bound to Smc1/3 is required . Scc2 homologs exist in fission yeast (Mis4), Drosophila, Xenopus, and human . By contrast, no homolog of Scc4 has been identified so far. We report here on the identification of fission yeast Ssl3 as a Scc4-like factor. Ssl3 is in complex with Mis4 and, as a bona fide loading factor, Ssl3 is required in G1 for cohesin binding to chromosomes but dispensable in G2 when cohesion is established. The discovery of a functional homolog of Scc4 indicates that the machinery of cohesin loading is conserved among eukaryotes.