Cohesin-dependent globules and heterochromatin shape 3D genome architecture in S. pombe.

Eukaryotic genomes are folded into three-dimensional structures, such as self-associating topological domains, the borders of which are enriched in cohesin and CCCTC-binding factor (CTCF) required for long-range interactions. How local chromatin interactions govern higher-order folding of chromatin fibres and the function of cohesin in this process remain poorly understood. Here we perform genome-wide chromatin conformation capture (Hi-C) analysis to explore the high-resolution organization of the Schizosaccharomyces pombe genome, which despite its small size exhibits fundamental features found in other eukaryotes. Our analyses of wild-type and mutant strains reveal key elements of chromosome architecture and genome organization. On chromosome arms, small regions of chromatin locally interact to form 'globules'. This feature requires a function of cohesin distinct from its role in sister chromatid cohesion. Cohesin is enriched at globule boundaries and its loss causes disruption of local globule structures and global chromosome territories. By contrast, heterochromatin, which loads cohesin at specific sites including pericentromeric and subtelomeric domains, is dispensable for globule formation but nevertheless affects genome organization. We show that heterochromatin mediates chromatin fibre compaction at centromeres and promotes prominent inter-arm interactions within centromere-proximal regions, providing structural constraints crucial for proper genome organization. Loss of heterochromatin relaxes constraints on chromosomes, causing an increase in intra- and inter-chromosomal interactions. Together, our analyses uncover fundamental genome folding principles that drive higher-order chromosome organization crucial for coordinating nuclear functions.

Synthetic heterochromatin bypasses RNAi and centromeric repeats to establish functional centromeres.

In the central domain of fission yeast centromeres, the kinetochore is assembled on CENP-A(Cnp1) nucleosomes. Normally, small interfering RNAs generated from flanking outer repeat transcripts direct histone H3 lysine 9 methyltransferase Clr4 to homologous loci to form heterochromatin. Outer repeats, RNA interference (RNAi), and centromeric heterochromatin are required to establish CENP-A(Cnp1) chromatin. We demonstrated that tethering Clr4 via DNA-binding sites at euchromatic loci induces heterochromatin assembly, with or without active RNAi. This synthetic heterochromatin completely substitutes for outer repeats on plasmid-based minichromosomes, promoting de novo CENP-A(Cnp1) and kinetochore assembly, to allow their mitotic segregation, even with RNAi inactive. Thus, the role of outer repeats in centromere establishment is simply the provision of RNAi substrates to direct heterochromatin formation; H3K9 methylation-dependent heterochromatin is alone sufficient to form functional centromeres.

Heterochromatin and RNAi are required to establish CENP-A chromatin at centromeres.

Heterochromatin is defined by distinct posttranslational modifications on histones, such as methylation of histone H3 at lysine 9 (H3K9), which allows heterochromatin protein 1 (HP1)-related chromodomain proteins to bind. Heterochromatin is frequently found near CENP-A chromatin, which is the key determinant of kinetochore assembly. We have discovered that the RNA interference (RNAi)-directed heterochromatin flanking the central kinetochore domain at fission yeast centromeres is required to promote CENP-A(Cnp1) and kinetochore assembly over the central domain. The H3K9 methyltransferase Clr4 (Suv39); the ribonuclease Dicer, which cleaves heterochromatic double-stranded RNA to small interfering RNA (siRNA); Chp1, a component of the RNAi effector complex (RNA-induced initiation of transcriptional gene silencing; RITS); and Swi6 (HP1) are required to establish CENP-A(Cnp1) chromatin on naïve templates. Once assembled, CENP-A(Cnp1) chromatin is propagated by epigenetic means in the absence of heterochromatin. Thus, another, potentially conserved, role for centromeric RNAi-directed heterochromatin has been identified.

In vivo levels of S-adenosylmethionine modulate C:G to T:A mutations associated with repeat-induced point mutation in Neurospora crassa.

In Neurospora crassa, the mutagenic process termed repeat-induced point mutation (RIP) inactivates duplicated DNA sequences during the sexual cycle by the introduction of C:G to T:A transition mutations. In this work, we have used a collection of N. crassa strains exhibiting a wide range of cellular levels of S-adenosylmethionine (AdoMet), the universal donor of methyl groups, to explore whether frequencies of RIP are dependent on the cellular levels of this metabolite. Mutant strains met-7 and eth-1 carry mutations in genes of the AdoMet pathway and have low levels of AdoMet. Wild type strains with high levels of AdoMet were constructed by introducing a chimeric transgene of the AdoMet synthetase (AdoMet-S) gene fused to the constitutive promoter trpC from Aspergillus nidulans. Crosses of these strains against tester duplications of the pan-2 and am genes showed that frequencies of RIP, as well as the total number of C:G to T:A transition mutations found in randomly selected am(RIP) alleles, are inversely correlated to the cellular level of AdoMet. These results indicate that AdoMet modulates the biochemical pathway leading to RIP.