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.