THE STABILITY OF NUCLEOSOMES AT THE REPLICATION FORK

Purified simian virus (SV40) minichromosomes were photoreacted with ps oralen under various conditions that moderately destabilize nucleosome s. This assay allows indirect distinction between stable nucleosomes, partially unravelled nucleosomes and nucleosomes containing (or lackin g) histone H1. In replicating molecules the passage of the replication machinery destabilizes the nucleosomal organization of the chromatin fiber over a distance of 650 to 1100 bp. In front of the fork, an aver age of two nucleosomes are destabilized presumably by the dissociation of histone H1 and the advancing replication machinery On daughter str ands, the first nucleosome is detected at a distance of about 260 nucl eotides from the elongation point. This nucleosome is interpreted to c ontain no histone H1, while no stepwise association of (H3-H4)(2) tetr amers with H2A/H2B dimers on nascent DNA can be detected in vivo. The second nucleosome after the replication fork appears to contain histon e H1. The prolonged nuclease sensitivity of newly replicated chromatin described in the literature therefore may not be due to a slow reasso ciation of histone H1. (C) 1996 Academic Press Limited