Structural and mechanistic insights into the MCM8/9 helicase complex.

MCM8 and MCM9 form a functional helicase complex (MCM8/9) that plays an essential role in DNA homologous recombination repair for DNA double-strand break. However, the structural characterization of MCM8/9 for DNA binding/unwinding remains unclear. Here, we report structures of the MCM8/9 complex using cryo-electron microscopy single particle analysis. The structures reveal that MCM8/9 is arranged into a heterohexamer through a threefold symmetry axis, creating a central channel that accommodates DNA. Multiple characteristic hairpins from the N-terminal oligosaccharide/oligonucleotide (OB) domains of MCM8/9 protrude into the central channel and serve to unwind the duplex DNA. When activated by HROB, the structure of MCM8/9's N-tier ring converts its symmetry from C3 to C1 with a conformational change that expands the MCM8/9's trimer interface. Moreover, our structural dynamic analyses revealed that the flexible C-tier ring exhibited rotary motions relative to the N-tier ring, which is required for the unwinding ability of MCM8/9. In summary, our structural and biochemistry study provides a basis for understanding the DNA unwinding mechanism of MCM8/9 helicase in homologous recombination.

Epstein-Barr virus protein BKRF4 restricts nucleosome assembly to suppress host antiviral responses.

Inhibition of host DNA damage response (DDR) is a common mechanism used by viruses to manipulate host cellular machinery and orchestrate viral life cycles. Epstein-Barr virus tegument protein BKRF4 associates with cellular chromatin to suppress host DDR signaling, but the underlying mechanism remains elusive. Here, we identify a BKRF4 histone binding domain (residues 15-102, termed BKRF4-HBD) that can accumulate at the DNA damage sites to disrupt 53BP1 foci formation. The high-resolution structure of the BKRF4-HBD in complex with a human H2A-H2B dimer shows that BKRF4-HBD interacts with the H2A-H2B dimer via the N-terminal region (NTR), the DWP motif (residues 80-86 containing D81, W84, P86), and the C-terminal region (CTR). The "triple-anchor" binding mode confers BKRF4-HBD the ability to associate with the partially unfolded nucleosomes, promoting the nucleosome disassembly. Importantly, disrupting the BKRF4-H2A-H2B interaction impairs the binding between BKRF4-HBD and nucleosome in vitro and inhibits the recruitment of BKRF4-HBD to DNA breaks in vivo. Together, our study reveals the structural basis of BKRF4 bindings to the partially unfolded nucleosome and elucidates an unconventional mechanism of host DDR signal attenuation.

Paraburkholderia caseinilytica sp. nov., isolated from the pine and broad-leaf mixed forest soil.

A novel Gram-stain-negative, aerobic, non-spore-forming, motile and rod-shaped bacterial strain, designated HM451T, was isolated from forest soil sampled at the Dinghushan Biosphere Reserve, Guangdong Province, PR China (112° 31' E 23° 10' N). It grew optimally at 28 °C, pH 5.0-6.0 and in the presence of 0-2.5 % (w/v) NaCl on R2A medium. Strain HM451T was closely related to Paraburkholderia mimosarum NBRC 106338T (98.6 % 16S rRNA gene sequence similarity), Paraburkholderia heleia NBRC 101817T (98.4 %) and Paraburkholderia silvatlantica SRMrh-20T (98.0 %). The 16S rRNA gene sequence analysis showed that strain HM451T and the three closely related strains formed a clade within the genus Paraburkholderia, but was clearly separated from the established species. The DNA-DNA relatedness value between strain HM451T and its phylogenetically closest relative, P. mimosarum NBRC 106338T, was much lower than 70 %. Strain HM451T contained ubiquinone 8 as the major respiratory quinone. Major fatty acids were C16 : 0, C17 : 0cyclo and summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c). The DNA G+C content of strain HM451T was 65.4 mol%. The major polar lipids were phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, two unidentified aminophospholipids, one unidentified aminolipid and a polar lipid. The phenotypic, chemotaxonomic and phylogenetic data showed that strain HM451T represents a novel species of the genus Paraburkholderia, for which the name Paraburkholderia caseinilytica sp. nov. is proposed. The type strain is HM451T (=GDMCC 1.1190T=LMG 30092T).