Deposition and Function of Histone H3 Variants in Tetrahymena thermophila

In Tetrahymena, HHT1 and HHT2 genes encode the same major histone H3; HHT3 and HHT4 encode similar minor H3 variants (H3s), H3.3 and H3.4. Green fluorescent protein (GFP)-tagged H3 is deposited onto chromatin through a DNA replication-coupled (RC) pathway. GFP-tagged H3.3 and H3.4 can be deposited both by a transcription-associated, replication-independent (RI) pathway and also weakly by an RC pathway. Although both types of H3s can be deposited by the RC pathway, DNA repair synthesis associated with meiotic recombination utilizes H3 specifically. The regions distinguishing H3 and H3.3 for their deposition pathways were identified. RC major H3 is not essential. Cells can grow without major H3 if the minor H3s are expressed at high levels. Surprisingly, cells lacking RI H3s are also viable and maintain normal nucleosome density at a highly transcribed region. The RC H3 is not detectably deposited by the RI pathway, even when there are no RI H3s available, indicating that transcription-associated RI H3 deposition is not essential for transcription. Minor H3s are also required to produce viable sexual progeny and play an unexpected role in the germ line micronuclei late in conjugation that is unrelated to transcription.