Segments of the genome enriched in repetitive sequences still present a challenge and are omitted in genome assemblies. For that reason, the exact composition of DNA sequences underlying the heterochromatic regions and the active centromeres are still unexplored for many organisms. The centromere is a crucial region of eukaryotic chromosomes responsible for the accurate segregation of genetic material. The typical landmark of centromere chromatin is the rapidly-evolving variant of the histone H3, CenH3, while DNA sequences packed in constitutive heterochromatin are associated with H3K9me3-modified histones. In the Pacific oyster Crassostrea gigas we identified its centromere histone variant, Cg-CenH3, that shows stage-specific distribution in gonadal cells. In order to investigate the DNA composition of genomic regions associated with the two specific chromatin types, we employed chromatin immunoprecipitation followed by high-throughput next-generation sequencing of the Cg-CenH3- and H3K9me3-associated sequences. CenH3-associated sequences were assigned to six groups of repetitive elements, while H3K9me3-associated-ones were assigned only to three. Those associated with CenH3 indicate the lack of uniformity in the chromosomal distribution of sequences building the centromeres, being also in the same time dispersed throughout the genome. The heterochromatin of C. gigas exhibited general paucity and limited chromosomal localization as predicted, with H3K9me3-associated sequences being predominantly constituted of DNA transposons.

Centromeres are chromosomal domains essential for kinetochore assembly and correct chromosome segregation. Inconsistent in their underlying DNA sequences, centromeres are defined epigenetically by the presence of the centromere-specific histone H3 variant CenH3. Most of the analyzed eukaryotes have monocentric chromosomes in which CenH3 proteins deposit into a single, primary constriction visible at metaphase chromosomes. Contrary to monocentrics, evolutionary sporadic holocentric chromosomes lack a primary constriction and have kinetochore activity distributed along the entire chromosome length. In this work, we identified cCENH3 protein, the centromeric H3 histone of the coleopteran model beetle Tribolium castaneum . By ChIP-seq analysis we disclosed that cCENH3 chromatin assembles upon a repertoire of repetitive DNAs. cCENH3 in situ mapping revealed unusually elongated T . castaneum centromeres that comprise approximately 40% of the chromosome length. Being the longest insect regional centromeres evidenced so far, T . castaneum centromeres are characterized by metapolycentric structure composed of several individual cCENH3-containing domains. We suggest that the model beetle T . castaneum with its metapolycentromeres could represent an excellent model for further studies of non-canonical centromeres in insects.

A series of cationic chlorido arene-iridium­(III) and arene-osmium­(II) complexes with bidentate pyridyl functionalized mesoionic carbenes (MIC) of the 1,2,3-triazol-5-ylidene type have been prepared. The variations in the ligand structures include the position of the pyridyl substituent relative to the triazolylidene ring (N-wingtip vs C-wingtip), phenyl versus ethyl substituents, and incorporation of several functional groups at the phenyl substituents. Five complexes have been characterized by X-ray structural analysis. All complexes, including osmium­(II) and ruthenium­(II) analogues having a pyrimidyl in place of the pyridyl group, have been studied for their cytotoxic activity on a human cervical carcinoma HeLa cell line. Two of the compounds, Ir 5 and Ir 9 , were the most cytotoxic with IC50 values of 7.33 μM and 2.01 μM, respectively. Examination of their cytotoxic effect on different cell lines revealed that they preferentially kill cancer over normal cells. The Ir 5 and Ir 9 compounds arrested cells in G2 and induced a dose-dependent increase in SubG0/G1 cell population. Apoptosis, as the primary mode of cell death, was confirmed by Annexin V/PI staining, detection of cleaved PARP, and caspases 3 and 7 activity upon treatment of HeLa cells with both compounds. The higher toxicity of Ir 9 is probably due to its increased accumulation in the cells compared to Ir 5 . The role of glutathione (GSH) in the protection of cells against Ir 5 and Ir 9 cytotoxicity was confirmed by pretreatment of cells either with buthionine sulfoximine (inhibitor of GSH synthesis) or N-acetyl-cysteine (precursor in GSH synthesis).

The red flour beetle Tribolium castaneum is an important pest of stored agricultural products and the first beetle whose genome was sequenced. So far, one high-copy-number and ten moderate-copy-number satellite DNAs (satDNAs) have been described in the assembled part of its genome. In this work, we aimed to catalog the entire collection of T. castaneum satDNAs. We resequenced the genome using Illumina technology and predicted potential satDNAs via graph-based sequence clustering. In this way, we discovered 46 novel satDNAs that occupied a total of 2.1% of the genome and were, therefore, considered low-copy-number satellites. Their repeat units, preferentially 140–180 bp and 300–340 bp long, showed a high A + T composition ranging from 59.2 to 80.1%. In the current assembly, we annotated the majority of the low-copy-number satDNAs on one or a few chromosomes, discovering mainly transposable elements in their vicinity. The current assembly also revealed that many of the in silico predicted satDNAs were organized into short arrays not much longer than five consecutive repeats, and some of them also had numerous repeat units scattered throughout the genome. Although 20% of the unassembled genome sequence masked the genuine state, the predominance of scattered repeats for some low-copy satDNAs raises the question of whether these are essentially interspersed repeats that occur in tandem only sporadically, with the potential to be satDNA “seeds”.