We have previously cloned the gene encoding a 115,000-Mr super T antigen (115K super T antigen), an elongated form of the Simian virus 40 large T antigen, originating from the rat cell line V 11 Fl clone 1, subclone 7 (May et al., J. Virol. 45:901-913, 1983). DNA sequence analysis has shown that the 115K super T antigen gene contains notably an in-phase duplication of a sequence located in the region of tsA mutations. We have also shown that the 115K super T antigen gene is able to induce the formation of transformed foci in transfected rat cells. After rat cell cultures were transfected with the cloned gene encoding 115K super T antigen, we obtained a large number of transformants as reported in this paper. In these transformants, we detected a very high frequency of new T antigen variants, as shown by immunoprecipitation of the cell extracts with anti-simian virus 40 tumor serum followed by electrophoresis in sodium dodecyl sulfatepolyacrylamide gels. Based on these results and all of the data presently available, it appears likely that the input plasmid or cosmid DNAs containing the cloned gene were first subjected to recombination events that yield new variant T antigen genes before these recombinant genes become integrated. The new variant T antigens observed in the transformants were predominantly those comigrating with normal-size large T antigen. In fact, these latter variants appeared to be indistinguishable from wild-type large T antigen as judged by restriction mapping by Southern blotting of the total genomic DNA of the transformants. Models of intermolecular or intramolecular homologous recombination occurring between or within the input plasmid or input cosmid DNA molecules are proposed to account for the formation of such revertants.Large chromosomal rearrangements have been found in many living organisms, notably in eucaryotes. These changes in chromosome structure (recombinations) include rearrangements, amplifications, deletions, and translocations of DNA segments. Such recombinations are involved (i) in the genome evolutiort (11), (ii) in the rearrangements of mammalian antibody genes during lymphocyte development (36), (iii) in the changes, mediated by mobile elements, in gene order and expression (36), and (iv) in the genesis of B cell-derived tumors in mice and humans (19).Similarly, after infection by simian virus 40 (SV40) or its DNA transfection into cultured mammalian cells, the SV40 genome frequently undergoes mutatibns and recombihations in these cells (5,30,38,41,42,48,49).In SV40-transformed rat or mouse cell lines, rearrangements of viral and adjacent cellular sequences occur during or after integration (2,4,9,16,26,28,34). The altered templates encode either shortened forms (truncated T antigens [7]) or elongated forms (super T antigens [8, 23, 25]) of large T antigen. The templates for truncated T antigens are interrupted in the distal exon of the SV40 early region, whereas the templates for super T antigen frequently contain in-phase tandem duplications of the tsA region (23, 25...
