Structure of integrated simian virus 40 DNA in transformed mouse cells

The structure of integrated viral DNA sequences in four lines of simian virus 40 (SV40)-transformed Balb/c 3T3 cells has been probed using restriction endonucleases and the Southern (1975) transfer method. By considering data from a large number of restriction digests of DNA from each line, and by using a novel method of handling the data, we have constructed fairly detailed physical maps of the integrated DNA in each line. The most striking of the features of the maps described here is that none is easily explained by the integration of a single SV40 genome into the DNA of the host cell. Three of the lines contain at least two distinct integrated segments and the fourth contains a single segment longer than the viral DNA. Considered individually, only two of the seven segments that we have mapped might be unit length. Of the remaining five, two are longer and three are shorter than the viral genome. It seems likely, therefore, that at least in SV40-transformed Balb/c 3T3 cells simple, single integrations are rare. The endpoints of these seven segments of integrated DNA fall at many positions distributed over the entire genome, confirming earlier studies (Ketner & Kelly, 1976; Botchan et al., 1976), which indicated that SV40 integration is not absolutely site-specific. Finally, one of the lines mapped here (SVB209) does not possess an intact SV40 early region, an observation that suggests the possibility that a normal viral large T polypeptide is not synthesized by this line.