HISTONE H1 MODULATES DNA-REPLICATION THROUGH MULTIPLE PATHWAYS IN XENOPUS EGG EXTRACT

We investigated the effects of histone His on DNA replication using Xe nopus egg extract, Mouse variants H1c and H1(0) were assembled onto Xe nopus sperm chromatin by the extract during the remodeling that accomp anies nuclear decondensation. The association of H1 with chromatin was rapid and concentration dependent, H1-associated chromatin displayed a typical nucleosomal repeat pattern indicating that linker histones a re properly positioned along the DNA, The presence of H1 on sperm chro matin reduced both the rate anti extent of DNA replication in egg extr act, This reduction in rate is due, in part, to a delay in initiation of replication within individual nuclei, Initiation in extract is depe ndent upon nuclear assembly, Analysis of the assembly process revealed that H1 does not inhibit nuclear membrane formation or the import of nuclear protein, however, it does slow the rate of nuclear lamina form ation, This H1-induced delay in lamina assembly is responsible for the delay in initiation as pre-assembled H1-containing nuclei initiate re plication at the same time as control nuclei, However, H1 inhibits rep lication even when lamina assembly is complete suggesting that H1 also affects replication directly, These data indicate that H1 modulates D NA replication through multiple pathways in egg extract.