STIMULATION OF REPLICATION EFFICIENCY OF A CHROMATIN TEMPLATE BY CHROMOSOMAL PROTEIN HMG-17

The effect of chromosomal protein HMG-17 on the replication of a chrom atin template was studied with minichromosomes containing the SV40 ori gin of replication. The minichromosomes were assembled from M13 DNA in Xenopus egg extracts in either the absence or presence of HMG-17. Str uctural data show that HMG-17 was efficiently incorporated into the ch romatin and induced an extended chromatin structure. Using an in vitro SV40 replication system, we find that minichromosomes containing HMG- 17 replicate with higher efficiency than minichromosomes deficient of HMG-17. The replicational potential of chromatin was enhanced only whe n HMG-17 was incorporated into the template during, but not after, chr omatin assembly. HMG-17 stimulated replication only from a chromatin t emplate, but not from protein-free DNA. Thus, HMG-17 protein enhances the rate of replication of a chromatin template by unfolding the highe r order chromatin structure and increasing the accessibility of target sequences to components of the replication machinery.