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Fig. 5 | Epigenetics & Chromatin

Fig. 5

From: Phylogenetic analysis of the core histone doublet and DNA topo II genes of Marseilleviridae: evidence of proto-eukaryotic provenance

Fig. 5

Evolutionary steps toward chromatinized replisomes in eukaryotes and Marseilleviridae. Two hypotheses concerning the evolution of the highly conserved replisome gene repertoires of MV genomes are presented. a In the late hypothesis, MV core histone doublets were derived by multiple horizontal gene transfer (HGT) events (blue arrows) from unspecified eukaryotic lineage(s) after eukaryotic diversification. This was followed by fusion of MV core histone genes and evolutionary divergence sufficient to erase sequence affinities with specific eukaryotic lineages as well as specific core histone variant clades. b In the early hypothesis, MV core histone doublet genes and DNA topoisomerase genes were acquired from a proto-eukaryotic ancestor encoding fused core histones prior to the evolutionary diversification of eukaryotic core histone variants for H2A and H3. The acquisition from an earlier point in proto-eukaryotic lineage would explain the presence of these genes as fusions because they stem from lineage lacking combinatorial core histone dimer interactions for H2B and H4. It would also explain why they were inherited as a single-core histone locus that evolved by ancient tandem duplications

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