Males and females of many animal species differ in their sex-chromosome karyotype, and this creates imbalances between X-chromosome and autosomal gene products that require compensation. Although distinct molecular mechanisms have evolved in three highly studied systems, they all achieve coordinate regulation of an entire chromosome by differential RNA-polymerase occupancy at X-linked genes. High-throughput genome-wide methods have been pivotal in driving the latest progress in the field. Here we review the emerging models for dosage compensation in mammals, flies and nematodes, with a focus on mechanisms affecting RNA polymerase II activity on the X chromosome.
Egelhofer TA*, Minoda A*, Klugman S*, Lee K, Kolasinska-Zwierz P, Alekseyenko AA, Cheung M-S, Day DS, Gadel S, Gorchakov AA, Gu T, Kharchenko PV, Kuan S, Latorre I, Linder-Basso D, Luu Y, Ngo Q, Perry M, Rechtsteiner A, Riddle NC, Schwartz YB, Shanower GA, Vielle A, Ahringer J, Elgin SCR, Kuroda MI, Pirrotta V, Ren B, Strome S, Park PJ**, Karpen GH**, Hawkins D**R, Lieb JD**. An assessment of histone-modification antibody quality. Nat Struct Mol Biol 2011;18(1):91-3.Abstract
We have tested the specificity and utility of more than 200 antibodies raised against 57 different histone modifications in Drosophila melanogaster, Caenorhabditis elegans and human cells. Although most antibodies performed well, more than 25% failed specificity tests by dot blot or western blot. Among specific antibodies, more than 20% failed in chromatin immunoprecipitation experiments. We advise rigorous testing of histone-modification antibodies before use, and we provide a website for posting new test results (http://compbio.med.harvard.edu/antibodies/).
DNA sequence variations in individual genomes give rise to different phenotypes within the same species. One mechanism in this process is the alteration of chromatin structure due to sequence variation that influences gene regulation. We composed a high-confidence collection of human single-nucleotide polymorphisms and indels based on analysis of publicly available sequencing data and investigated whether the DNA loci associated with stable nucleosome positions are protected against mutations. We addressed how the sequence variation reflects the occupancy profiles of nucleosomes bearing different epigenetic modifications on genome scale. We found that indels are depleted around nucleosome positions of all considered types, whereas single-nucleotide polymorphisms are enriched around the positions of bulk nucleosomes but depleted around the positions of epigenetically modified nucleosomes. These findings indicate an increased level of conservation for the sequences associated with epigenetically modified nucleosomes, highlighting complex organization of the human chromatin.
The Drosophila melanogaster male-specific lethal (MSL) complex binds the single male X chromosome to upregulate gene expression to equal that from the two female X chromosomes. However, it has been puzzling that approximately 25% of transcribed genes on the X chromosome do not stably recruit MSL complex. Here we find that almost all active genes on the X chromosome are associated with robust H4 Lys16 acetylation (H4K16ac), the histone modification catalyzed by the MSL complex. The distribution of H4K16ac is much broader than that of the MSL complex, and our results favor the idea that chromosome-wide H4K16ac reflects transient association of the MSL complex, occurring through spreading or chromosomal looping. Our results parallel those of localized Polycomb repressive complex and its more broadly distributed chromatin mark, trimethylated histone H3 Lys27 (H3K27me3), suggesting a common principle for the establishment of active and silenced chromatin domains.
The male-specific lethal (MSL) complex upregulates the single male X chromosome to achieve dosage compensation in Drosophila melanogaster. We have proposed that MSL recognition of specific entry sites on the X is followed by local targeting of active genes marked by histone H3 trimethylation (H3K36me3). Here we analyze the role of the MSL3 chromodomain in the second targeting step. Using ChIP-chip analysis, we find that MSL3 chromodomain mutants retain binding to chromatin entry sites but show a clear disruption in the full pattern of MSL targeting in vivo, consistent with a loss of spreading. Furthermore, when compared to wild type, chromodomain mutants lack preferential affinity for nucleosomes containing H3K36me3 in vitro. Our results support a model in which activating complexes, similarly to their silencing counterparts, use the nucleosomal binding specificity of their respective chromodomains to spread from initiation sites to flanking chromatin.