Small and middle T antigens contribute to lytic and abortive polyomavirus infection

Using three different polyomavirus hr-t mutants and two polyomavirus mlT mutants, the authors studied induction of S-phase by mutants and wild-type virus in quiescent mouse kidney cells, mouse 3T6 cells, and FR 3T3 cells. At different times after infection, they measured the proportion of T-antigen-positive cells, the incorporation of [3H]thymidine, the proportion of DNA-synthesizing cells, and the increase in total DNA, RNA, and protein content of the cultures. In permissive mouse cells, they also determined the amount of viral DNA and the proportion of viral capsid-producing cells. In polyomavirus hr-t mutant-infected cultures, the onset of host DNA replication was delayed by several hours, and a smaller proportion of T-antigen-positive cells entered S-phase than in wild-type-infected cultures. Of the two polyomavirus mlT mutants studied, dl-23 behaved similarly to wild-type virus in many, but not all, parameters tested. The poorly replicating but well-transforming mutant dl-8 was able to induce S-phase, and (in permissive cells) progeny virus production, in only about one-third of the T-antigen-positive cells. From the experiments, the authors concluded that mutations affecting small and middle T-antigen cause a reduction in the proportion of cells responding to virus infection and a prolongation of the early phase, i.e., the period before cells center S-phase. In hr-t mutant-infected mouse 3T6 cells, production of viral DNA was <10% of that in wild-type-infected cultures; low hr-t progeny production in 3T6 cells was therefore largely due to poor viral DNA replication