Studies on the initiation and elongation reactions in the simian virus 40 DNA replication system.

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Abstract
The synthesis of oligoribonucleotides by DNA primase in the presence of duplex DNA containing the simian virus 40 (SV40) origin of replication was examined. Small RNA chains (10-15 nucleotides) were synthesized in the presence of the four common ribonucleoside triphosphates, SV40 large tumor antigen (T antigen), the human DNA polymerase .alpha. (pol .alpha.)-DNA primase complex, the human single-stranded DNA-binding protein (HSSB), and topoisomerase I isolated from HeLa cells. The DNA primase-catalyzed reaction showed an absolute requirement for T antigens, HSSB, and pol .alpha.. The requirement for HSSB was not satisfied by other SSBs that can support the T-antigen-catalyzed unwinding of DNA containing the SV40 origin of replication. Oligoribonucleotide synthesis occurred with a lag that paralleled the lag observed in DNA synthesis. These results indicate that the specificity for the HSSB in the SV40 replication reaction is due to the pol .alpha.-primase-mediated synthesis of the Okazaik fragments. In contrast to this specificity, the elongation of Okazaki fragments can be catalyzed by a variety of different DNA polymerases, including high level of pol .alpha., the polymerase .delta. holoenzyme, T4 polymerase holoenzyme, the Escherichia coli polymerase III holoenzyme, and other polymerases. These observations suggest that leading-strand synthesis in the in vitro SV40 replication system can be nonspecific.