A mutant of the defective SV40-adenovirus 7 hybrid, PARA, induces the synthesis of SV40 large T-antigen (T-ag) that is not transported to the nucleus and accumulates in the cytoplasm of infected or transformed cells. The effect of this cT mutation on the transport of wild-type (WT) T-ag was examined by coinfection experiments. Immunofluorescence staining for T-ag revealed that when SV40-transformed green monkey kidney cells were infected with the cT mutant of PARA [PARA(2cT)] there was a total loss of nuclear T-ag (nT-ag) reactivity concomitant with the appearance of cytoplasmic T-ag (CT-ag) reactivity. The dominance of the cT-ag phenotype was also manifest in cells coinfected with PARA(2cT) and either SV40 or WT PARA, both of which induce nT-ag during single infection. Two lines of evidence indicate that the absence of nuclear T-ag reactivity in coinfected cells is due to a failure of migration of WT T-ag into the nucleus, rather than an inhibition of its synthesis. First, coinfection of cells with SV40 and either helper adenovirus or PARA-adenovirus populations, at the multiplicities of infection employed in these studies, did not reduce the yields of infectious SV40. Second, cells coinfected with PARA(2cT) and deletion mutants of SV40 which encode T-ag polypeptides of reduced molecular weight expressed the cT-ag phenotype, while the presence of the deleted forms of T-ag was confirmed by immunoprecipitation and SDS-polyacrylamide gel electrophoresis. The observation that the replication of infectious SV40 is not inhibited in cells coinfected with PARA(2cT) and expressing the cT phenotype indicates that levels of nuclear T-ag below the limits of detection by immunofluorescence are sufficient to promote SV40 DNA synthesis. The dominant effect of the cT mutation was specific for transport of SV40 T-ag, since normal migration of adenovirus tumor and virion antigens as well as SV40 virion antigen occurred. Several possible mechanisms for the dominance of the CT-ag phenotype are presented.
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