Other H3 replacement variants

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]. 

Bbd was discovered as an H2A-like protein encoded by human ESTs [77]. H2A.Bbd is only 42% identical to conventional H2A and is smaller due to a shortened C-terminus that lacks the ubiquitination site that is present in most other H2As (Fig. 6). H2A.Bbd appears to be associated with nucleosomes as indicated by the co-purification of an epitope-tagged H2A.Bbd with nucleosomal fragments in a sucrose gradient run at high ionic strength [77]. 

Messenger RNAs for H2A.Bbd have been detected in human testis, fibroblasts and lymphocytes [77], but little is known about the amount of H2A.Bbd protein in these or other human cell types or about its presence in other species. The distribution of H2A.Bbd in chromatin was examined by expressing epitope-tagged or GFP-tagged H2A.Bbd in cultured cells. These studies revealed a striking deficiency of H2A.Bbd in the inactive X chromosome, leading to its name which stands for Histone H2A Barr-body deficient [77]. The distribution of H2A.Bbd overlapped extensively with that of H4 acetylated on lysine 12, suggesting that H2A.Bbd may preferentially associate with transcriptionally active chromatin. 

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