G the presence of nine discrete nanochromosomes (see Additional file 2: Figure
G the presence of nine discrete nanochromosomes (see Additional file 2: Figure S2A). We had earlier included the protein sequences of these nanochromosomes in a study about the evolutionary history of histone H3 in eukaryotes [12]. Two of the nine nanochromosomes, HIS32A and HIS32B, encoded almost identical proteins. The single difference was H3.2aS29/H3.2bL29 (see Additional file 3: Figure S3). Another almost identical histone H3 variant was H3.1, encoded by HIS31, which had H3.1S29/C122 instead of A112 in H3.2a/b. We previously proposed that all Stylonychia H3 variants had evolved from a H3.3-like ancestor [12]. H3.3 and H3.5, encoded by HIS33 and HIS35, were most reminiscent of H3.3 in Hydra or nuclearids, which resembled the putative ancestral protoH3 [12]. Further, both H3.4 and H3.6, encoded by HIS34 or HIS36, respectively, were closely related to H3.3. The only variant containing a GT-AG-type intron was HIS33. In contrast to these variants, whose coding sequence (CDS) size was between 411 and 417 nucleotides (nt) with predicted molecular weights of 15.25 to 15.75 PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/26104484 kDa (Table 1), two more deviant variants had evolved. H3.7 (gene HIS37) had a predicted size of 20.01 kDa and consisted of 543 nt in thecoding region. Most of the deviations in H3.7 occurred within the N-terminus. Of similar size was H3.8 (predicted size 20.48 kDa). Deviations in H3.8 were also found in the N-terminus, and additional residues were attached to the C-terminus. BLAST searches using the Stylonychia macronuclear genome draft database (http://stylo.ciliate.org/) provided no evidence for further H3 variants. Strikingly, the most prominent differences between these variants occurred within sequence motifs known to be targets of chromatin-modifying enzymes. These motifs included all the above residues adjacent to H3K27, and also the similar motif adjacent to H3K9 (see Additional file 3: Figure S3; referring to numbering in Hydra histone H3). Unless otherwise indicated, we ignore the correct numbering of Stylonychia H3 variant residues, which is often deviant, to ease comparability between homologous motifs. A complete similarity matrix of these homologous motifs with correct numbering is provided in Figure 1A. Lysine-27 was conserved in all histone H3 variants, and lysine-9 in almost all of these variants, except H3.7. At least two main groups may be relevant, which contained either AKK27S (H3.1, H3.2) or ARK27S/T. Notably, serine-10, which is usually conserved in animal H3.3, was not found in most Stylonychia H3 variants, except in H3.8 within the ASK26S motif. By contrast, H3K27 was accompanied by serine or threonine in almost all variants, except H3.7 (ARK61M). The motif adjacent to H3K36 (GVK36K-PHR) was identical to animal H3 in H3.1 to H3.5 and almost identical in H3.6, but it deviated at homologous loci of H3.7 and H3.8. Interestingly, a very similar motif had evolved in H3.7 (LVK105KLPFQ), directly before the N-terminal end of the 1 helix adjacent to the histone fold domain. The H3K4 motif (ARTK4QT) did not differ from animals in H3.1 to H3.6, except in H3.7 and H3.8. The transfer and deposition of histone variants into chromatin is Enasidenib mechanism of action mediated via their association with specific histone chaperones. For example, Asf1 is involved in the transfer of H3-H4 dimers, and acts as a donor for the variant specific chaperone complexes CAF-1 (replicationdependent; specificity for H3.1-H4 dimers) or HIRA (replication-independent; specificity for H3.3-H4 dimers) [29]. Th.
