Deposition of the major histone H3 (H3.1) is coupled to DNA synthesis during DNA replication and possibly DNA repair, whereas histone variant H3.3 serves as the replacement variant for the DNA-synthesis-independent deposition pathway. To address how histones H3.1 and H3.3 are deposited into chromatin through distinct pathways, we have purified deposition machineries for these histones. The H3.1 and H3.3 complexes contain distinct histone chaperones, CAF-1 and HIRA, that we show are necessary to mediate DNA-synthesis-dependent and -independent nucleosome assembly, respectively. Notably, these complexes possess one molecule each of H3.1/H3.3 and H4, suggesting that histones H3 and H4 exist as dimeric units that are important intermediates in nucleosome formation. This finding provides new insights into possible mechanisms for maintenance of epigenetic information after chromatin duplication.

Studies that concern the mechanism of DNA replication have provided a major framework for understanding genetic transmission through multiple cell cycles. Recent work has begun to gain insight into possible means to ensure the stable transmission of information beyond just DNA, and has led to the concept of epigenetic inheritance. Considering chromatin-based information, key candidates have arisen as epigenetic marks, including DNA and histone modifications, histone variants, non-histone chromatin proteins, nuclear RNA as well as higher-order chromatin organization. Understanding the dynamics and stability of these marks through the cell cycle is crucial in maintaining a given chromatin state.