In eukaryotic nuclei, DNA is coiled around histone proteins to form nucleosomes. The pattern by which nucleosomes are compacted into higher-order structures determines the accessibility of chromatin and hence it’s transcriptional activity. Many different factors, including the linker histone H1, histone modifications, chromatin remodelling enzymes and non-histone proteins play important roles in structuring chromatin. Various classes of RNA have also been implicated in regulating the higher-order structure of chromatin. Among many examples; Argonaute associated small silencing RNAs are known to sometimes exert their inhibitory effects by directing histone modifications or DNA methylation and lncRNAs have been shown to serve as cis-acting scaffolds coordinating the action of histone-modifying enzymes. It’s been known for decades that RNA makes up a proportion of chromatin, but exactly what types of RNAs and what their roles are is not yet clear. A new paper in Molecular Cell (Schubert et al.) sets out to answer these questions, characterising chromatin-associated RNAs in Drosophila and finding an important role for RNA in regulating chromatin compaction and accessibility.
Schubert et al. found that RNAs were involved in maintaining the accessibility of chromatin using an assay in which chromatin is digested by a nuclease (DNase). This digestion creates a ladder of DNA of different sizes on a gel, ranging from single nucleosomal fragments to far larger pieces. The extent of digestion is dependent on the level of chromatin condensation; the more compacted the nucleosomal structure, the more refractory it will be to DNase digestion. The researchers found that incubation of chromatin with an RNase prior to DNase treatment resulted in more compacted chromatin; DNase digestion was less efficient. Using different RNases and inhibitors they discovered that the RNA population involved was single-stranded and synthesised by RNA polymerase II.
Similar results were found when the authors used density-gradient centrifugation to isolate chromatin. They found a fraction of RNA associated with chromatin (caRNA) that when digested, resulted in chromatin becoming more compacted and shifting to higher density fractions. Interestingly, this RNA-dependent chromatin condensation effect is reversible; when the compacted chromatin was extracted and incubated with fresh cellular extracts, it reopens, again migrating in lighter fractions, re-associated with RNA. This caRNA-dependent chromatin accessibility is also dependent on chromatin-associated proteins, as it could not be rescued under denaturing conditions.
Using mass spectrometry, the authors identified 59 proteins that had lower affinity for chromatin after removal of RNA. One of these was the highly abundant chromatin decondensation factor 31 (Df31). A study that I have long intended to write about (Filion et al. 2010) used the binding of Df31 and 52 other chromatin associated proteins to determine that rather than dividing Drosophila chromatin into two types: transcriptionally repressed heterochromatin and active euchromatin, we should instead think in terms of five different classes which they colour coded. Green, blue and black chromatin are broadly transcriptionally repressed, whilst red and yellow are euchromatic and more transcriptionally active. Df31 is found bound to these red and yellow types of more open chromatin. Schubert et al found that Df31 chromatin binding is stabilised by caRNAs. Df31 binds histone H3 in the absence of RNA, but its affinity is substantially enhanced by the addition of RNA. RNAi knockdown of Df31 causes a fraction of genomic DNA to be more compacted.
Deep sequencing of the pool of caRNAs revealed that they were enriched for non-coding RNAs, especially a class termed small nucleolar RNAs (snoRNAs). snoRNAs are known to guide the modification of bases in ribosomal, transfer and messenger RNAs. They have also been implicated in RNA editing and splicing. Schubert et al. found that 30 of the 186 snoRNAs expressed in Drosophila embryos were found associated with chromatin. Using fluorescent in situ hybridisation, they showed that two of the most highly enriched snoRNAs could be visualised binding to the interbands of Drosophila polytene chromosomes (ie. euchromatin). These two snoRNAs were able to ‘rescue’ compacted chromatin in the density-gradient experiments, and were shown to directly interact with Df31.
The authors also found that extracts of human cells could rescue RNase treated compacted chromatin, and that snoRNAs are found associated with chromatin in human cells.
Schubert et al have therefore characterised a novel conserved role for snoRNAs mediating the accessibility of higher-order chromatin structures. As none of the protein components known to complex with snoRNAs in snoRNPs were identified in the mass spectrometry experiments, it appears that snoRNAs form distinct ribonucleoprotein complexes to mediate this chromatin associated role. Df31 is one such important interactor, linking chromatin-associated snoRNAs and histone proteins within nucleosomes to maintain accessibility of red and yellow chromatin. However, the knockdown of Df31 resulted in far more limited and localised chromatin compaction than RNase treatment, suggesting that the role of caRNAs goes beyond the maintenance of open euchromatin, but also to regulating heterochromatic accessibility. Seeing as 58 other chromatin proteins, as well as many other snoRNAs and other caRNAs, were implicated in RNA-associated chromatin accessibility functions, this initial study has just revealed the tip of an iceberg. Just how this RNA-chromatin network functions to maintain accessibility requires a lot of work. As with many of the best studies, the light shone serves to partially illuminate the scale of our ignorance.
Schubert T, Pusch MC, Diermeier S, Benes V, Kremmer E, Imhof A, & Längst G (2012). Df31 Protein and snoRNAs Maintain Accessible Higher-Order Structures of Chromatin. Molecular cell PMID: 23022379
Filion GJ, van Bemmel JG, Braunschweig U, Talhout W, Kind J, Ward LD, Brugman W, de Castro IJ, Kerkhoven RM, Bussemaker HJ, & van Steensel B (2010). Systematic protein location mapping reveals five principal chromatin types in Drosophila cells. Cell, 143 (2), 212-24 PMID: 20888037