Chromatin maturation depends on continued DNA-replication

Chromatin remodeling factors and DNA replication

Chromatin structures have to be precisely duplicated during DNA replication to maintain tissue-specific gene expression patterns and specialized domains, such as the centromeres. Chromatin remodeling factors are key components involved in this process and include histone chaperones, histone modifying enzymes and ATP-dependent chromatin remodeling complexes. Several of these factors interact directly with components of the replication machinery. Histone variants are also important to mark specific chromatin domains. Because chromatin remodeling factors render chromatin dynamic, they may also be involved in facilitating the DNA replication process through condensed chromatin domains.

Replication of SV40 minichromosomes in vitro

We operationally define two forms of SV40 minichromosomes, a 75S-form, prepared at low salt concentration, referred to as native minichromosomes, and a 50S-form, obtained after treatment with 0.5 M potassium acetate, the salt-treated minichromosomes. Both preparations of minichromosomes serve well as templates for replication in vitro. Their respective replication products are strikingly different: replicated native minichromosomes contain a densely packed array of the maximal number of nucleosomes whereas replicated salt-treated minichromosomes carry, on average, half of the maximal number. We conclude that in both cases parental nucleosomes are transferred to progeny DNA, and, in addition, that an assembly of new nucleosomes occurs during the replication of native minichromosomes. This is apparently due to the presence of a nucleosome assembly factor as a constituent of native minichromosomes that dissociates upon treatment with salt. We further show that preparations of minichromosomes usually contain significant amounts of copurifying hnRNP particles and SV40 virion precursor particles. However, these structures do not detectably affect the replication and the chromatin assembly reactions.

Chromatin structure and DNA replication

The structure of (3H) thymidine pulse labeled, replicating chromatin differs from that of non replicating chromatin by several operational criteria which are related to the higher nuclease sensitivity of replicating chromatin. We summarize the structural changes that we observe using replicating chromatin as substrate for micrococcal nuclease. The data suggest a more extended configuration of replicating compared to non replicating chromatin. We use these data to discuss a model of the chromatin structure in the vicinity of replication forks. Finally, we present data to show that the reversion of structural changes in replicative chromatin depends on continued DNA replication.