SUMMARYChinese hamster kidney cells are semi-permissive to simian virus 40 (SV4o). Exposure to mitomycin C (MC) of Chinese hamster kidney cells infected with SV4o DNA enhanced the yield of infectious virus Io-to zoo-fold. This stimulation occurred whether the treatment was performed before or after infection. A simultaneous increase in the number of V antigen-synthesizing cells and virus-producing cells, as well as the virus burst size, was observed upon MC pretreatment, whereas the proportion of T antigen-synthesizing cells remained unchanged. MC pretreatment clearly stimulated virus DNA replication in SV4o virus-infected cells. Cells treated with MC exhibited an unbalanced growth pattern, with continuing protein synthesis in the absence of cell division and a markedly reduced ability to replicate the cellular DNA. These results suggest that MC enhances the permissiveness of Chinese hamster kidney cells by inducing the synthesis of a specific cellular factor(s) required for SV4o replication in these cells. Exposure to ultraviolet light also enhanced infectious virus production in Chinese hamster kidney cells.

SUMMARYTreatment of a fully permissive monkey kidney cell clone (CV1Cll) with 5-iodoz'-deoxyuridine (IdUrd) before infection with SV4o virus enhances the yield of virions 1o-to 5o-fold. The increase is detectable only after slowing down the virus growth cycle by reducing the m.o.i, and by incubating at low temperature. The IdUrd pre-treatment enhances SV4o DNA replication and the number of Vantigen and virus-synthesizing cells. The potentiating effect of IdUrd is not observed when the pre-treated cells are infected with SV4o DNA. The synthesis of SV4o T-antigen is increased even in the presence of cytosine arabinoside (Ara-C). IdUrd inhibits cellular DNA synthesis but the incorporation of 3H-uridine and 3H-leucine into RNA and proteins is not affected. Late virus functions are preferentially expressed in the cells in which cellular DNA synthesis is inhibited. The results suggest that the enhancement by IdUrd of SV4o replication would be the consequence of at least two complementary events: (0 stimulation of an early virus function localized between the arrival of the virus DNA in the nucleus and T-antigen induction; (2) inhibition of cellular DNA synthesis with a consequent greater availability of cellular factors required for virus growth.

SUMMARYA number of chemical and physical agents were screened to determine their effectiveness in inducing simian virus (SV4o) production in a virogenic clone of SV4o-transformed Chinese hamster cells. Mitomycin C (MC) was the most effective inducing agent, and MC induction was further characterized. It was found that levels of infectious SV4o DNA were increased above control levels as early as 6 h after addition of MC to the culture medium and reached maximum levels by 48 h. Virus capsid (V) antigen and virions followed with a lag of about 24 h. V antigen production was sensitive to hydroxyurea, suggesting a dependence on virus DNA synthesis. The proportion of virus-producing cells (infectious centres) and the virus burst per cell were both stimulated by MC. Studies of 3H-thymidine incorporation demonstrated that the rate of SV4o DNA synthesis was maximal at 48 h post-induction, at which time cellular DNA synthesis was ahnost abolished. Caffeine, at doses not toxic to non-induced cells, strongly inhibited SV4o production in both noninduced and induced cells, suggesting some role for DNA repair mechanisms.

Throughout in vitro passages, Chinese hamster kidney (CHK) cells progressively lost susceptibility to SV 40 virus infection while remaining continuously susceptible to viral DNA infection. Upon infection with SV 40 virus or viral DNA, the CHK cell line supported viral DNA and virus replication at a low level. SV 40 transformed CHK cell lines spontaneously produced small amounts of viral DNA and virions. The percentage of virus-producing cells was low. Various clones derived from each of these lines behaved as the parental cell population, leading to the conclusion that each CHK cell, whether transformed or not with SV 40, is potentially permissive for this virus.

A somatic cell hybrid (Cl. 6d) was originated from the fusion of mouse 3T3-4E) and spontaneous yielder SV 40-transformed Chinese hamster (CHK/SVLP AG) cells. During the early stages of its history, the C1. 6d hybrid underwent a rapid chromosome loss, preferentially loosing hamster chromosomes. This was not a constant tendency of the hybrid cells. As the parental CHK)SVLP AG cells, the hybrid cells were always found 100 per cent SV40 T-antigen positive. While CHK/SVLP AG cells infectious SV 40 DNA, V-antigen and virus were regularly detected, in the hybrid cells only infectious DNA was occasionally detected. This was not due either to the loww of an essential Chinese hamster gene(s) or to the presence of an inhibiting mouse cell component(s); it was apparently the consequence of inability of the cells to properly activate the resident SV 40 genome(s). After superinfection with SV 40 DNA, the hybrid cells-though capable of synthesizing SV 40 V-antigen--were unable to ensure virus assembly. Experimental evidence was obtained suggesting that SV 40 maturation is dependent of a cellular function(s).