Cellular and SV40 Chromatin: Replication, Segregation, Ubiqiritination, Nuclease-hypersensitive Sites, HMG-containing Nueleosomes, and Heterochromatin-specific Protein

Abstract

During lytic infection, SV40 DNA exists as a minichromosome within the host nucleus and is replicated in a manner similar to that of the cellular genome. The 5.2-kb circular, double-stranded SV40 DNA directs site-specific synthesis of a single replication "bubble," which is then enlarged through movement of both replication forks (for review, see DePamphilis and Wassarman 1980). Separate, covalently closed, circular SV40 DNA molecules appear approximately 5 minutes after the forks meet halfway around the genome (Levine et al. 1970). We have recently discovered a new class of SV40 replicative intermediates participating in these latest stages of SV40 DNA replication (Sundin and Varshavsky 1980, 1981). All members of this class are catenated dimers, two circular molecules of SV40 DNA linked topologically by one or more intertwining events. The catenated SV40 DNA dimers comprise three specific families and occur as minichromosomes. They are all rapidly processed in vivo to mature supercoiled SV40 DNA I. We have also found conditions for selective arrest of SV40 replication in vivo at the stage of catenated chromosomes (Sundin and Varshavsky 1981). SV40 is the first example of a circular, double-stranded replicon that uses multiply intertwined catenated dimers as segregation intermediates. It is possible that analogous mechanisms operate during the final stages of replication in a variety of other eukaryotic and prokaryotic replicons. Some of the early studies on singly intertwined catenated dimers in mitochondrial, SV40, and other DNAs are those done by Rush et al. (1970), Jaenisch and Levine (1972), Novick et al. (1973), and Kupersztoch and Helinski (1973). What follows is a condensed account of the "DNA-level" work on multiply intertwined catenated dimers (Sundin and Varshavsky 1980, 1981), as well as our more recent data on the chromatin organization of SV40 chromosomes during the segregation of daughter DNA molecules.

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