Biology subjects

Toner, C. M.

Publications and source records attributed to Toner, C. M..

3 recordsLinked to original sources

bioRxiv2025

Melbournevirus encodes a shorter H2B-H2A doublet histone variant that forms structurally distinct nucleosome structures.

Unique among viruses, some giant viruses utilize histones to organize their genomes into nucleosomes. Melbournevirus encodes a distinct H2B-H2A histone doublet variant in addition to the canonical H4-H3 and H2B-H2A doublets. This viral histone variant has a truncated H2B portion and its amino acid sequence deviates from that of the main viral H2B-H2A throughout the entire coding region. It is less abundant than the main H2B-H2A doublet, is essential for melbournevirus fitness, and is conserved in all Marseilleviridae. The cryo-EM structure of a nucleosome-like particle reconstituted with this H2B-H2A variant and viral H4-H3 reveals that only 90 base pairs of DNA are stably bound, significantly less than in eukaryotic nucleosomes and viral nucleosomes made with the main fused viral histone doublets. The reduced ability to bind DNA can be attributed to structural differences between variant and main H2B-H2A. Variant melbournevirus nucleosomes are less stable, possibly aiding rapid genome unpacking to initiate gene expression. Our results highlight the remarkable propensity of giant viruses to appropriate the utility of histones for their specialized purposes.

molecular biology↗

bioRxiv2024

Characterization of Medusavirus encoded histones reveals nucleosome-like structures and a unique linker histone

The organization of DNA into nucleosomes is a ubiquitous and ancestral feature that was once thought to be exclusive to the eukaryotic domain of life. Intriguingly, several representatives of the Nucleocytoplasmic Large DNA Viruses (NCLDV) encode histone-like proteins that in Melbournevirus were shown to form nucleosome-like particles. Medusavirus medusae (MM), a distantly related giant virus, encodes all four core histone proteins and, unique amongst most giant viruses, a putative acidic protein with two domains resembling linker histone H1. Here we report the structure of nucleosomes assembled with Medusavirus histones and highlight similarities and differences with eukaryotic and Melbournevirus nucleosomes. Our structure provides insight into how variations in histone tail and loop lengths are accommodated within the context of the nucleosome. We show that Medusavirus histones assemble into tri-nucleosome arrays, and that the putative linker histone H1 does not function in chromatin compaction. These findings expand our understanding of viral histones and suggest that Medusavirus histones represent a snapshot in the evolutionary timeline of nucleosome architecture. ONE SENTENCE SUMMARYThe four Medusavirus medusae core histones form nucleosome-like structures that combine features of eukaryotic and other viral nucleosomes.

molecular biology↗

bioRxiv2021

Melbournevirus-encoded histone doublets are recruited to virus particles and form destabilized nucleosome-like structures

The organization of genomic DNA into defined nucleosomes has long been viewed as a hallmark of eukaryotes. This paradigm has been challenged by the identification of minimalist histones in archaea, and more recently by the discovery of genes that encode fused remote homologs of the four eukaryotic histones in Marseilleviridae, a subfamily of giant viruses that infect amoebae. We demonstrate that viral doublet histones localize to the cytoplasmic viral factories after virus infection, and ultimately to mature virions. CryoEM structures of viral nucleosome-like particles show strong similarities to eukaryotic nucleosomes, despite the limited sequence identify. The unique connectors that link the histone chains contribute to the observed instability of viral nucleosomes, and some histone tails assume structural roles. Our results further expand the range of organisms that have nucleosomes and suggest a specialized function of histones in the biology of these unusual viruses. One Sentence SummarySome large DNA viruses encode fused histone doublets that are targeted to viral factories and assemble into open nucleosome-like structures.

molecular biology↗