Discussion

Discussion

In mammalian cells, about 2 m of DNA are packaged within an interphase nucleus that is about 10-20 µm in diameter. This architectural feat is achieved through several layers of DNA coiling. The first level of DNA packaging is the wrapping of 147 bp of DNA into 1.65 superhelical turns around the surface of an octamer of histone proteins to create the nucleosome core particle. This octamer is itself a tripartite structure composed of a central (H3-H4)2 tetramer flanked by two H2A-H2B dimers. In higher eukaryotes, nucleosome core particles are joined together by variable lengths of linker DNA (roughly between 18 and 65 bp). Linker histones (a family of proteins related to histone H1) shield the excess negative charge of the linker DNA to promote folding of chromatin into a higher-order structure known as the 30-nm fibre. In proliferating cells, the duplication of chromatin structure during DNA replication is achieved through two concerted reactions (Bench et al, 1995).

First, DNA strand unwinding by the DNA helicase (a complex of six MCM proteins) and DNA synthesis by the polymerase require the transient disruption of parental histone-DNA contacts and the transfer of parental histones (often referred to as pre-existing or old histones) behind the replication fork. The accuracy and efficiency of parental histone transfer are likely important to prevent spontaneous DNA damage caused by frequent pausing of replication forks and the loss of parental histone post-translational modifications (PTMs) involved in epigenetic silencing of gene expression . In addition to parental histone transfer, histone chaperones rapidly deposit newly synthesised histones onto nascent DNA through a nucleosome assembly pathway that is tightly linked to DNA synthesis. Unlike chaperones that facilitate protein folding, histone chaperones promote de novo nucleosome assembly by shielding the excess positive charge of histones (Rasheed, 1989).

At physiological ionic strength, this histone charge shielding is necessary to prevent the formation of aggregates between histones and DNA. The purpose of this review is to describe the roles of histone chaperones in DNA synthesis-coupled nucleosome assembly. In addition, eukaryotic cells have also evolved pathways that deposit either canonical core histones or histone variants onto DNA outside of S-phase. We will also describe the implication of these replication-independent nucleosome assembly pathways in transcription and other processes that require genome-wide changes in DNA packaging, namely cellular senescence and chromatin assembly of the paternal genome.

Histone chaperones assemble nucleosomes during DNA replication and repair. The importance of these chaperones is underscored by the observation that cellular DNA is prone to damage when nucleosome assembly is incomplete. Specialized histone chaperones, such as nucleoplasmin (Np), Drosophila NLP (dNLP), N1, and TAF1, play significant roles in oocytes, embryos, and somatic cells

Anti-silencing function 1 (ASF1) is a histone chaperone involved in both replication-coupled and replication-independent nucleosome assembly. Through an affinity purification strategy, ASF1 was recently co-purified from human cell chromatin in a stable complex with the MCMs and histone H3-H4 . Some of the histones co-purified with the ASF1-MCM complex likely are parental histones because they contain PTMs (H3 K9 ...