Reconstitution of wild type viral DNA in simian cells transfected with early and late SV40 defective genomes

DNA sequences outside the simian virus 40 early region cause downregulation of T-antigen production in permissive simian cells

Using a series of modified wtSV40 and early region SV40 DNAs we assayed the effect of viral late region sequences on T-antigen production by the SV40 early region. We found that SV40 late region (L-SV40) DNA sequences reduced T-antigen (T-Ag) production by the SV40 early region (E-SV40) when both viral regions were linked as they are in wtSV40 DNA. This was demonstrated by Western analysis which showed that E-SV40 DNA produced 10 times more T-Ag than wtSV40 DNA L-SV40, with its own promoter but unlinked to E-SV40 DNA, also greatly inhibited T-Ag production when it was contrasfected with E-SV40. Therefore, L-SV40 DNA inhibited T-Ag production by E-SV40 DNA when present in cis or in trans. We have shown that expression of the SV40 late transcription unit dominated that of the early (T-Ag gene) transcription unit because late region RNA accumulated to much higher levels than early viral RNA. However, in contrasfected cells L-SV40 DNA did not replicate to higher levels than E-SV40 DNA. We offer a model for control of T-Ag expression in which a relatively small amount of T-Ag activates late transcription at the expense of T-Ag gene transcription and that this represents a switch from early to late viral gene expression. We suggest that when activation of the late transcription unit occurs at the late promoter, expression of the T-Ag gene is greatly reduced. The L-SV40 promoter may inhibit T-Ag gene transcription by sequestering cellular factors required for early transcription, factors which may be present in limited amounts. We suggest further that activation of late transcription allows for the necessary production of large amounts of capsomeres and virions and downregulation of early transcription prevents the early region from interfering with capsid synthesis. We tested the model using a construct with a wild-type T-Ag gene but with mutations in the SV40 major late promoter which prevent the promoter from being bound by cellular repressors of late transcription. We found that this construct, which overproduces late SV40 RNA, was defective for T-Ag production. This indicates that activation of the late promoter results in repression of T-Ag gene expression.

Persistence of the SV40 early region without expression in permissive simian cells

SV40 containing recombinant vectors were introduced into permissive simian, non-permissive rodent and semi-permissive human cell lines, and assayed for transformation. All mouse and human cell clones expressed T-antigen (T-Ag) and were morphologically transformed when they contained only the wt T-Ag gene (E-SV40) or the entire wt viral genome with an interrupted late region. However, of 63 simian clones with these recombinant vectors, none became morphologically transformed and T-Ag containing cells were rare or absent. Nearly all simian cell lines made either no detectable early SV40 RNA or only small amounts of viral RNA but contained viral DNA restriction fragments similar to those in the original recombinant vectors. Functional T-Ag genes were recoverable from several cell clones and used to regenerate infectious virus. Hence, T-Ag gene expression had been suppressed. We found two conditions where T-Ag expression was activated. In a BSC-1 cell line containing E-SV40 DNA, subsequent introduction of a vector with a functional viral late coding region (L-SV40) resulted in the appearance of T-Ag and transformation. These findings suggest that L-SV40 sequences activate or enhance T-Ag expression and that this activation requires a functional Vpl gene. We found also, that vectors with E-SV40 DNA from the bipartite variant EL-SV40 consistently transformed simian CV-1 cells. Transformation was shown to be effected by the multiple alterations present in the regulatory region of this variant.