Project description:As one of the most commonly recognized sub-cellular structures, mitotic chromosome structure and assembly is still lacking. Except the identified proteins and well known genomic composition, the identified mitotic chromosome RNA species is limited. We used a targeted protocol based on 5â-tag sequencing to profile mouse 3T3 cell mitotic chromosome associated RNA composition.
Project description:Mitotic chromosomes are one of the most commonly recognized sub-cellular structures in eukaryotic cells. Yet basic information necessary to understand their structure and assembly, such as their composition, is still lacking. Recent proteomic studies have begun to fill this void, identifying hundreds of RNA-binding proteins bound to mitotic chromosomes. However, by contrast, there are only two RNA species (U3 snRNA and rRNA) that are known to be associated with the mitotic chromosome, suggesting that there are many mitotic chromosome-associated RNAs (mCARs) not yet identified. Here, using a targeted protocol based on 5'-tag sequencing to profile the mammalian mCAR population, we report the identification of 1279 mCARs, the majority of which are ncRNAs, including lncRNAs that exhibit greater conservation across 60 vertebrate species than the entire population of lncRNAs. There is also a significant enrichment of snoRNAs and specific SINE RNAs. Finally, ∼40% of the mCARs are presently unannotated, many of which are as abundant as the annotated mCARs, suggesting that there are also many novel ncRNAs in the mCARs. Overall, the mCARs identified here, together with the previous proteomic and genomic data, constitute the first comprehensive catalogue of the molecular composition of the eukaryotic mitotic chromosomes.
Project description:Recent studies have shown that repressive chromatin machinery, including DNA methyltransferases (DNMTs) and Polycomb Repressor Complexes (PRCs), bind to chromosomes throughout mitosis and their depletion results in increased chromosome size. Enzymes that catalyse H3K9 methylation, such as Suv39h1, Suv39h2, G9a, GLP are also retained by mitotic chromosomes. Surprisingly however, mutants lacking H3K9me3 have unusually small and compact mitotic chromosomes that are associated with increased H3S10ph and H3K27me3 levels. Chromosome size and centromere compaction in these mutants was rescued by providing exogenous Suv39h1, or inhibiting Ezh2 activity. Quantitative proteomic comparisons of native mitotic chromosomes isolated from wildtype versus Suv39h1,2 double null ESCs revealed that H3K9me3 was essential for the efficient retention of bookmarking factors such as Esrrb. These results highlight an unexpected role for repressive heterochromatin domains in preserving transcription factor binding through mitosis, and underscore the importance of H3K9me3 for sustaining chromosome architecture and epigenetic memory during cell division.
Project description:Condensin drives mitotic chromosome assembly by folding chromatin into loops and is enriched in the vicinity of highly expressed genes, but the significance of such proximity with respect to condensin activity has remained unclear. Here, by modulating the occupancy of RNA Pol II in vivo, we show that transcription plays no role in the steady state association of condensin with DNA. Rather, transcription stalls and even displaces condensin, hindering its ability to fold chromatin and to support chromosome segregation. Our results highlight a key aspect of the integrated functioning of condensin and suggest that a tight control of transcription underlies mitotic chromosome assembly.
Project description:During mitosis chromosomes reorganise into highly compact, rod-shaped forms, thought to consist of consecutive chromatin loops around a central protein scaffold. Condensin complexes are involved in chromatin compaction, but the contribution of other chromatin proteins, DNA sequence and histone modifications is less understood. A large region of fission yeast DNA inserted into a mouse chromosome was previously observed to adopt a mitotic organisation distinct from that of surrounding mouse DNA. Here we show that a similar distinct structure is common to a large subset of insertion events in both mouse and human cells and is coincident with the presence of high levels of heterochromatic H3 lysine 9 trimethylation (H3K9me3). Hi-C and microscopy indicate that the heterochromatinised fission yeast DNA is organised into smaller chromatin loops than flanking euchromatic mouse chromatin. We conclude that heterochromatin alters chromatin loop size, thus contributing to the distinct appearance of heterochromatin on mitotic chromosomes, such as at centromeres.
Project description:During mitosis, the genome is restructured to facilitate chromosome segregation, accompanied by dramatic changes in gene expression. However, the mechanisms that underlie mitotic transcriptional regulation are unclear. In contrast to transcribed genes, centromere regions retain transcriptionally active RNA Polymerase II (RNAPII) in mitosis. Here, we demonstrate that chromosome-localized cohesin is necessary and sufficient to retain active RNAPII on mitotic centromeres. Failure to remove cohesin from mitotic chromosome arms dramatically alters mitotic gene expression, and results in a failure to release elongating RNAPII and nascent transcripts from mitotic chromosomes. We propose that prophase cohesin removal is the key step in reprogramming gene expression as cells transition from G2 to mitosis, and is temporally coupled with chromosome condensation to coordinate chromosome segregation with changes in gene expression.
Project description:During mitosis, the genome is restructured to facilitate chromosome segregation, accompanied by dramatic changes in gene expression. However, the mechanisms that underlie mitotic transcriptional regulation are unclear. In contrast to transcribed genes, centromere regions retain transcriptionally active RNA Polymerase II (RNAPII) in mitosis. Here, we demonstrate that chromosome-localized cohesin is necessary and sufficient to retain active RNAPII on mitotic centromeres. Failure to remove cohesin from mitotic chromosome arms dramatically alters mitotic gene expression, and results in a failure to release elongating RNAPII and nascent transcripts from mitotic chromosomes. We propose that prophase cohesin removal is the key step in reprogramming gene expression as cells transition from G2 to mitosis, and is temporally coupled with chromosome condensation to coordinate chromosome segregation with changes in gene expression.