Cis-Effects of Heterochromatin on Heterochromatic and Euchromatic Gene Activity in Drosophila Melanogaster

Chromosomal rearrangements that juxtapose heterochromatin and euchromatin can result in mosaic inactivation of heterochromatic and euchromatic genes. This phenomenon, position effect variegation (PEV), suggests that heterochromatic and euchromatic genes differ in their regulatory requirements. This report describes a novel method for mapping regions required for heterochromatic genes, and those that induce PEV of a euchromatic gene. P transposase mutagenesis was used to generate derivatives of a translocation that variegated for the light(+) (lt(+)) gene and carried the euchromatic white(+) (w(+)) gene on a transposon near the heterochromatin-euchromatin junction. Cytogenetic and genetic analyses of the derivatives showed that P mutagenesis resulted in deletions of several megabases of heterochromatin. Genetic and molecular studies showed that the derivatives shared a euchromatic breakpoint but differed in their heterochromatic breakpoint and their effects on seven heterochromatic genes and the w(+) gene. Heterochromatic genes differed in their response to deletions. The lt(+) gene was sensitive to the amount of heterochromatin at the breakpoint but the heterochromatic 40Fa gene was not. The severity of variegated w(+) phenotype did not depend on the amount of heterochromatin in cis, but varied with local heterochromatic environment. These data are relevant for considering mechanisms of PEV of both heterochromatic and euchromatic genes.     

组蛋白变异体H2A.Z的研究进展

H2A.Z是组蛋白H2A的变异体之一,是高度保守的组蛋白变异体,参与保护常染色体,防止形成异染色质;并且与转录调节、抗沉默、沉默和基因组稳定性有关。组蛋白变异体H2A.Z可能与染色体形成独立的结构域,从而调节染色质结构功能。但是,H2A.Z对染色体结构功能的作用机制还不是很清楚。组蛋白变异体H2A.Z和它的表观遗传修饰对染色体动态结构和功能起重要的作用。该文将对组蛋白变异体H2A.Z进行综述。     

基于组蛋白甲基化信息的H2A.Z和H2A核小体识别

组蛋白H2A的变体H2A.Z在基因的表达过程中发挥着重要的作用。根据H2A.Z和H2A核小体中组蛋白甲基化修饰方式的不同,作者应用多样性增量二次判别方法(increment of diversity with quadratic discriminant,IDQD)成功地对H2A.Z和H2A核小体进行了识别,说明了以组蛋白甲基化信息作为特征参数的IDQD模型对H2A.Z和H2A核小体识别的有效性。通过计算DNA序列的柔性,发现H2A.Z核小体对应的DNA序列的平均柔性比常规H2A核小体对应的DNA序列的平均柔性弱。     

H2A.Z.2.2 is an alternatively spliced histone H2A.Z variant that causes severe nucleosome destabilization

The histone variant H2A.Z has been implicated in many biological processes, such as gene regulation and genome stability. Here, we present the identification of H2A.Z.2.2 (Z.2.2), a novel alternatively spliced variant of histone H2A.Z and provide a comprehensive characterization of its expression and chromatin incorporation properties. Z.2.2 mRNA is found in all human cell lines and tissues with highest levels in brain. We show the proper splicing and in vivo existence of this variant protein in humans. Furthermore, we demonstrate the binding of Z.2.2 to H2A.Z-specific TIP60 and SRCAP chaperone complexes and its active replication-independent deposition into chromatin. Strikingly, various independent in vivo and in vitro analyses, such as biochemical fractionation, comparative FRAP studies of GFP-tagged H2A variants, size exclusion chromatography and single molecule FRET, in combination with in silico molecular dynamics simulations, consistently demonstrate that Z.2.2 causes major structural changes and significantly destabilizes nucleosomes. Analyses of deletion mutants and chimeric proteins pinpoint this property to its unique C-terminus. Our findings enrich the list of known human variants by an unusual protein belonging to the H2A.Z family that leads to the least stable nucleosome known to date.     

Removal of H2A.Z by INO80 promotes homologous recombination

The mammalian INO80 remodelling complex facilitates homologous recombination (HR), but the mechanism by which it does this is unclear. Budding yeast INO80 can remove H2A.Z/H2B dimers from chromatin and replace them with H2A/H2B dimers. H2A.Z is actively incorporated at sites of damage in mammalian cells, raising the possibility that H2A.Z may need to be subsequently removed for resolution of repair. Here, we show that H2A.Z in human cells is indeed rapidly removed from chromatin flanking DNA damage by INO80. We also report that the histone chaperone ANP32E, which is implicated in removing H2AZ from chromatin, similarly promotes HR and appears to work on the same pathway as INO80 in these assays. Importantly, we demonstrate that the HR defect in cells depleted of INO80 or ANP32E can be rescued by H2A.Z co‐depletion, suggesting that H2A.Z removal from chromatin is the primary function of INO80 and ANP32E in promoting homologous recombination.     

A conserved function for the H2A.Z C terminus

Histone H2A variants generate diversity in chromatin structure and functions, as nucleosomes containing variant H2A histones have altered physical, chemical, and biological properties. H2A.Z is an evolutionarily ancient and highly conserved H2A variant that regulates processes ranging from gene expression to the DNA damage response. Here we find that the unstructured portion of the C-terminal tail of H2A.Z is required for the normal functions of this histone variant in budding yeast. We have also identified a novel splice isoform of the human H2A.Z-2 gene that encodes a C-terminally truncated H2A.Z protein that is similar to the truncation mutants we identified in yeast. The short forms of H2A.Z in both yeast and human cells are more loosely associated with chromatin than the full-length proteins, indicating a conserved function for the H2A.Z C-terminal tail in regulating the association of H2A.Z with nucleosomes.