Tetrahymena macronucleus

H3 replacement variants are also present in plants [108] and Tetrahymena [109]. Phylogenetic analyses indicates that these H3 replacement variants arose independently of animal H3.3 [109]. Like H3.3, the plant H3 replacement variant also appears to be preferentially deposited into transcriptionally active chromatin [110]. The H3 replacement variant in Tetrahymena, hv2, is found only in the transcriptionally active macronucleus [49]. In contrast to H3.3, the amino acid differences between hv2 and the replication dependent H3 of Tetrahymena do not appear to be essential for replication independent incorporation into chromatin [111]. In this case constitutive expression appears to be the dominant factor that can drive replication independent deposition of hv2 or an H3 variant that is normally replication dependent. This difference between hv2 and H3.3 is not necessarily surprising because hv2 appears to have arisen independently of H3.3 and does not have the structural features characteristic of H3.3 [109]. 

Allis, C.D., Glover, C.V.C., Bowen, J.K., and Gorovsky, M.A. (1980) Histone variants specific to the transcriptionally active amitotically dividing macronucleus of the unicellular eukaryote, Tetrahymena thermophila. Cell 20, 609-617. 

H2A.1, H2A.2, and H2A.X are phosphorylated at serine residue 1 [8,9]. H2A.Z is not phosphorylated. In vitro protein kinase C phosphorylates H2A at serine residue 1 [10]. Telrahymena H2A is phosphorylated in the C-terminal sequence [11]. Tetrahymena H2A.1 is phosphorylated at serine residues 122, 124, and 129, while H2A.2 is modified at serine residues 122 and 128 (Fig. 3). Phosphorylation of H2A occurs in the transcriptionally active macronucleus of Tetrahymena thermophila, but not in the transcriptionally inert micronucleus [12]. Tetrahymena H2A variant hvl is phosphorylated [13]. H4, like H2A, is phosphorylated at N-terminal serine... 

In a supplementary pathway, links between histone H3 Lys4 methylation and the upregulation of RNA synthesis have also been made. This discrete modification colocalizes with acetylated histone residues and is enriched in the transcriptionally active macronucleus of Tetrahymena [194]. Histone methylation at H3 Lys4 has been recently attributed to the novel HMT SET9, which contains the conserved SET catalytic domain, and noticeably lacks the juxtaposed pre- and post-SET... 

Except for histone H4, each of the other histone types are found in different isoforms and are called histone variants. The chapter by Pehrson (Chapter 8) will provide a more in-depth discussion of these forms. It is worth pointing out that with regard to transcription through nucleosomes, some of these variants are expressed in a replication-independent process and are found in active gene fractions that have been prepared using the nuclease-sensitive solubilization procedures described above. Of particular note are two minor histone variants, H2A.Z and H3.3. Both are expressed throughout the cell cycle and incorporated into the nucleosomes of active genes ([38,39], see reviews [46,47]). For example, both Tetrahymena, H2A.Z (termed Tetrahymena hvl) and an H3.3-like histone (hv2) are preferentially present in the active macronucleus and are expressed in the micronucleus just prior to the time when this nucleus becomes transcriptionally active [48,49]. Suto et al. [50] have determined the crystal structure of a nucleosome... 

McDonald, B. B. 1962. Synthesis of deoxyribonucleic acid by micro-and macronucleus of Tetrahymena pyriformis. J. Cell Biol., 13 193-203. 

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