Chromatin structure contribution to the synaptonemal complex formation

Meiosis is a key cellular and molecular process for sexual reproduction contributing to the genetic variability of organisms. This process takes place after DNA replication and consists in a double cellular division, giving rise to four haploid daughter cells or gametes. Meiotic recombination between homologous chromosomes, in the meiotic prophase I, is mediated by a tripartite structure named Synaptonemal Complex (SC). The SC is a peptidic scaffold in which the chromatin of homologous chromosomes is organized during the pachytene stage, holding chromosomes together until the meiotic recombination and genetic exchange have taken place. The role of chromatin structure in formation of the SC and the meiotic recombination at meiotic prophase I remain largely unknown. In this review we address the epigenome contribution to the SC formation at meiotic prophase I, with particular attention on the chromatin structure modifications occurring during the sub-stages of meiotic prophase I. Received 18 September 2008; received after revision 10 October 2008; accepted 24 October 2008     

Roles of the Major Apoptotic Nuclease-DNA Fragmentation Factor-in Biology and Disease

It has now been more than ten years since the discovery of the major apoptotic nuclease, DNA fragmentation factor (DFF), also known as caspaseactivated DNase (CAD). Here we review the recent literature that has uncovered new insight into DFF’s regulation, and both its positive and negative roles in human disease. Cells from mice deficient in DFF still undergo apoptotic death without significant cellautonomous DNA degradation. Their corpses’ genomes are subsequently degraded by lysosomal DNase II after phagocytosis. However,DFF-deficient mice are more susceptible to cancer. Indeed, several different cancers in humans are associated with defects in DFF expression and it has been proposed that DFF is a p53-independent tumor suppressor. Negative aspects of DFF expression include contributing to susceptibility to acquire systemic lupus erythematosus, to chromosomal translocations that result in mixed lineage leukemias, and in the possible spreading of oncogenes and HIV due to horizontal gene transfer. Received 06 August 2008; received after revision 03 September 2008; accepted 09 September 2008     

Epigenetic mechanisms in mammals

DNA and histone methylation are linked and subjected to mitotic inheritance in mammals. Yet how methylation is propagated and maintained between successive cell divisions is not fully understood. A series of enzyme families that can add methylation marks to cytosine nucleobases, and lysine and arginine amino acid residues has been discovered. Apart from methyltransferases, there are also histone modification enzymes and accessory proteins, which can facilitate and/or target epigenetic marks. Several lysine and arginine demethylases have been discovered recently, and the presence of an active DNA demethylase is speculated in mammalian cells. A mammalian methyl DNA binding protein MBD2 and de novo DNA methyltransferase DNMT3A and DNMT3B are shown experimentally to possess DNA demethylase activity. Thus, complex mammalian epigenetic mechanisms appear to be dynamic yet reversible along with a well-choreographed set of events that take place during mammalian development.     

The role of the nuclear envelope in cellular organization

Over the last years it has become evident that the nuclear envelope (NE) is more than a passive membrane barrier that separates the nucleus from the cytoplasm. The NE not only controls the trafficking of macromolecules between the nucleoplasm and the cytosol, but also provides anchoring sites for chromosomes and cytoskeleton to the nuclear periphery. Targeting of chromatin to the NE might actually be part of gene expression regulation in eukaryotes. Mutations in certain NE proteins are associated with a diversity of human diseases, including muscular dystrophy, neuropathy, lipodistrophy, torsion dystonia and the premature aging condition progeria. Despite the importance of the NE for cell division and differentiation, relatively little is known about its biogenesis and its role in human diseases. It is our goal to provide a comprehensive view of the NE and to discuss possible implications of NE-associated changes for gene expression, chromatin organization and signal transduction. Received 8 August 2005; received after revision 13 October 2005; accepted 13 October 2005     

偃麦草Ee染色体组特异DNA片段的分离、定位与应用

以中间偃麦草(Thinopyrum intermedium)、二倍体长穗偃麦草(Lophopyrum elongatum)、拟鹅观草(Pseudoroegneria spicata)和6个小麦对照为材料筛选240条10碱基RAPD引物. 结果发现引物H4在二倍体长穗偃麦草中扩增出1条长为740 bp的高拷贝DNA(记做Ee740),而小麦则扩增不出该片段.序列比对结果显示,Ee740位于散布重复序列与LTR反转录转座子athila之间,且仅与小麦基因组一个212 bp的片段有87%的同源性,因此Ee740应为一新DNA序列.根据Ee740设计特异PCR引物H4-F和 H4-R,对含E染色体组的材料和小麦材料进行扩增,结果显示,仅中间偃麦草和长穗偃麦草能扩增出Ee740.进而对1套中国春-倍体长穗偃麦草附加系(CS1E-S7E)进行扩增,结果发现仅CS2E、CS4E、CS5E、CS7E能扩增出Ee740, 即Ee740分布在偃麦草的2E、4E、5E和7E染色体上.进一步对小麦茸毛偃麦草的后代材料进行SCAR分析,结果表明Ee740可以用于小麦背景中Ee染色质的检测.     

Single molecule DNA compaction by purified histones

The compaction of single DNA molecules by purified histones is studied using magnetic tweezers. The compaction rate increases rapidly when the histone concentration is increased from 0.002 to 0.2 mmol/L, and saturates when the concentration is beyond 0.2 mmol/L. The time course of compaction is exponential at low histone concentrations. It becomes sigmoidal at high concentrations. Cooperativity between the histones bound to DNA is proposed to be responsible for the transition. The histones are loaded onto DNA randomly at low concentrations. They tend to bind DNA cooperatively at high con-centrations because the structural torsions of DNA induced by the bound histones become overlapping so that the binding of one histone facilitates the binding of others. Under very large forces, the com- pacted histone-DNA complex can be disrupted in a discrete manner with a step size of -60 nm. But the histones cannot be completely stripped off DNA, as is revealed by the lowered B-S transition plateau of the histone-bound DNA.     

Chromatosomes are not produced fromTetrahymena chromatin by micrococcal nuclease digestion

Summary The subnucleosomal organization ofTetrahymena chromatin, which has an unusual H1 histone, was investigated by NaCl extraction and micrococcal nuclease digestion of nuclei. It was found thatTetrahymena histone H1 is extracted with 0.35 M NaCl, whereas bovine thyroid H1 is not. Micrococcal nuclease digestion ofTetrahymena nuclei did not yield chromatosomes as a stable intermediate, whereas digestion of bovine thyroid nuclei did.