Polyoma virus minichromosomes: a soluble in vitro replication system.

Polyoma virus minichromosomes were isolated from infected 3T6 cells by hypotonic extraction of isolated nuclei. The kinetics of in vitro DNA synthesis in the nuclear extract was similar to that observed with intact nuclei. The majority of the products of in vitro DNA synthesis sedimented with replicative intermediate (RI) minichromosomes and migrated as two bands (RI-a and RI-b) on 1.4% agarose gels. The kinetics of deoxynucleotide monophosphate incorporation into these species was consistent with the existence of several rate-limiting steps in in vitro replication by polyoma minichromosomes. Electron microscope analysis showed that the RI-a band consisted almost entirely of RI theta structures ranging from 46 to 87% replicated, with one-half of all theta structures 67 +/- 4% replicated. The RI-b material was more complex, consisting of sigma and alpha structures with tails ranging from 7 to 114% of polyoma genome length and, less frequently, of linked and multiple linked dimeric structures.