Histone H4K20 tri‐methylation at late‐firing origins ensures timely heterochromatin replication

Among other targets, the protein lysine methyltransferase PR‐Set7 induces histone H4 lysine 20 monomethylation (H4K20me1), which is the substrate for further methylation by the Suv4‐20h methyltransferase. Although these enzymes have been implicated in control of replication origins, the specific contribution of H4K20 methylation to DNA replication remains unclear. Here, we show that H4K20 mutation in mammalian cells, unlike in Drosophila, partially impairs S‐phase progression and protects from DNA re‐replication induced by stabilization of PR‐Set7. Using Epstein–Barr virus‐derived episomes, we further demonstrate that conversion of H4K20me1 to higher H4K20me2/3 states by Suv4‐20h is not sufficient to define an efficient origin per se, but rather serves as an enhancer for MCM2‐7 helicase loading and replication activation at defined origins. Consistent with this, we find that Suv4‐20h‐mediated H4K20 tri‐methylation (H4K20me3) is required to sustain the licensing and activity of a subset of ORCA/LRWD1‐associated origins, which ensure proper replication timing of late‐replicating heterochromatin domains. Altogether, these results reveal Suv4‐20h‐mediated H4K20 tri‐methylation as a critical determinant in the selection of active replication initiation sites in heterochromatin regions of mammalian genomes.     

Trans-inactivation: Repression in a wrong place

Trans-inactivation is the repression of genes on a normal chromosome under the influence of a rearranged homologous chromosome demonstrating the position effect variegation (PEV). This phenomenon was studied in detail on the example of brown^Dominant allele causing the repression of wild-type brown gene on the opposite chromosome. We have investigated another trans-inactivation-inducing chromosome rearrangement, In(2)A4 inversion. In both cases, brown^Dominant and In(2)A4, the repression seems to be the result of dragging of the euchromatic region of the normal chromosome into the heterochromatic environment. It was found that cis-inactivation (classical PEV) and trans-inactivation show different patterns of distribution along the chromosome and respond differently to PEV modifying genes. It appears that the causative mechanism of trans-inactivation is de novo heterochromatin assembly on euchromatic sequences dragged into the heterochromatic nuclear compartment. Trans-inactivation turns out to be the result of a combination of heterochromatin-induced position effect and the somatic interphase chromosome pairing that is widespread in Diptera.     

The karyotype of Nothoscordum arenarium Herter (Gilliesioideae, Alliaceae): A populational and cytomolecular analysis

The genus Nothoscordum Kunth comprises approximately 20 species native to South America. Karyologically, the genus is remarkable for its large chromosomes and Robertsonian translocations. Variation in chromosome number has been recorded in a few polyploid species and it is unknown among diploids. This study presents the chromosome number and morphology of 53 individuals of seven populations of N. arenarium Herter (2n = 10). In addition, karyotype analyses after C-banding, staining with CMA and DAPI, and in situ hybridization with 5S and 45S rDNA probes were performed in six individuals from one population. All individuals exhibited 2n = 10 (6M + 4A), except for one tetraploid (2n = 20, 12M + 8A) and one triploid (2n = 15, 9M + 6A) plant. C-banding revealed the presence of CMA(+) /DAPI (-) heterochromatin in the short arm and in the proximal region of the long arm of all acrocentric chromosomes. The 45S rDNA sites co-localized with the CMA (+) regions of the acrocentrics short arms, while the 5S rDNA probe only hybridized with the subterminal region of a pair of metacentric chromosomes. A change in the pattern of CMA bands and rDNA sites was observed in only one individual bearing a reciprocal translocation involving the long arm of a metacentric and the long arm of an acrocentric chromosome. These data suggest that, despite isolated cases of polyploidy and translocation, the karyotype of N. arenarium is very stable and the karyotypic instability described for other species may be associated with their polyploid condition.     

Karyotype and nuclear DNA content of hexa-, octo-, and duodecaploid lines of Bromus subgen. Ceratochloa

The subgenus Ceratochloa of the genus Bromus includes a number of closely related allopolyploid forms or species that present a difficult taxonomic problem. The present work combines data concerning chromosome length, heterochromatin distribution and nuclear genome size of different 6x, 8x and 12x accessions in this subgenus. Special attention is paid to the karyotype structure and genomic constitution of duodecaploid plants recently found in South America. Hexaploid lineages possess six almost indistinguishable genomes and a nuclear DNA content between 12.72 pg and 15.10 pg (mean 1Cx value = 2.32 pg), whereas octoploid lineages contain the same six genomes (AABBCC) plus two that are characterized by longer chromosomes and a greater DNA content (1Cx = 4.47 pg). Two duodecaploid accessions found in South America resemble each other and apparently differ from the North American duodecaploid B. arizonicus as regards chromosome size and nuclear DNA content (40.00 and 40.50 pg vs. 27.59 pg). These observations suggest that the South American duodecaploids represent a separate evolutionary lineage of the B. subgenus Ceratochloa, unrecognized heretofore.     

组蛋白去乙酰化酶抑制剂(HDAIs)对异染色质的作用

组蛋白乙酰化和去乙酰化可调节染色体的多种功能,例如基因表达和染色体分离等。研究发现,组蛋白去乙酰化酶抑制剂(histone deacetylase inhibitors,HDACIs)可诱导分化、生长阻断和肿瘤细胞凋亡,目前HDACIs正作为抗肿瘤药物进行临床试验,在肿瘤治疗中显示出具有较好的应用前景。然而,人们对于HDACIs在生物体内是如何发挥作用以及不同类型细胞为何会有不同的应答途径却关注甚少。本综述通过讨论HDACIs对周期和非周期细胞中组蛋白去乙酰化酶的抑制结果,来阐明组蛋白乙酰化模式的动力学特征,特别是对基因组异染色质的作用。     

Use of C-banding for identifying radiation induced micronuclei

Differentiation of micronuclei (MN) caused by ionizing radiation from those caused by chemicals is a crucial step for managing treatment of individuals exposed to radiation. MN in binucleated lymphocytes in peripheral blood are widely used as biomarkers for estimating dose of radiation, but they are not specific for ionizing radiation. MN induced by ionizing radiation originate predominantly as a result of chromosome breaks (clastogenic action), whereas MN caused by chemical agents are derived from the loss of entire chromosomes (aneugenic action). C-banding highlights centromeres, which might make it possible to distinguish radiation induced MN, i.e., as a byproduct of acentric fragments, from those caused by the loss of entire chromosomes. To test the use of C-banding for identifying radiation induced MN, a blood sample from a healthy donor was irradiated with 3 Gy of Co-60 gamma rays and cultured. Cells were harvested and dropped onto slides, divided into a group stained directly with Giemsa and another processed for C banding, then stained with Giemsa. The frequency of MN in 500 binucleated cells was scored for each method. In preparations stained with Giemsa directly, the MN appeared as uniformly stained structures, whereas after C banding, some MN exhibited darker regions corresponding to centromeres that indicated that they were not derived from acentric fragments. The C-banding technique enables differentiation of MN from acentric chromosomal material. This distinction is useful for improving the specificity of the MN assay as a biomarker for ionizing radiation.