Using three different polyomavirus hr-t mutants and two polyomavirus mlT mutants, we 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, we 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, we also determined the amount of
viral DNA and the proportion of viral capsid-producing cells. In polyomavirus hr-t mutant-infected cultures, 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 our experiments, we conclude 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 enter S-phase. In hr-t mutant-infected mouse 3T6 cells, production of
viral DNA was less than 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.