Genomic rearrangements in a mouse cell line containing integrated SV40 DNA.

In the SV40-transformed mouse embryo fibroblast cell line SVT2/S, genomic rearrangements involving the SV40 DNA and flanking host sequences were identified by Southern blot hybridization using viral DNA as probe. No rearrangements of SV40 DNA integrated into nonpermissive mouse cells have been previously described. The standard arrangement found in the majority of subclones was mapped with 20 restriction enzymes, 10 of which cleave sites within the SV40 DNA. A single copy of a defective integrated viral genome is present, in which the late region is missing from about nucleotide 200 clockwise to about nucleotide 1750. The rest of the viral genome including the origin of replication and T antigen binding region is present and colinear with SV40 DNA, except for an internal repeat of about 1750 bp located between nucleotides 2750 and 4500. Rearrangements were found in 4 out of 20 random subclones of the parental SVT2/S cell line and 3 of the 4 continued to rearrange. The thioguanine-resistant cell line 281-1-4, derived from SVT2/S, remained stable on subculture but a chloramphenicol-resistant mutant, 107-6-4, derived from 281-1-4, was highly unstable. In 107-6-4, unique rearrangements were found in 6 of 31 subclones of a population that had undergone abut 25 doublings from a single-cell isolate. The high rate of rearrangement and the sporadic expression of rearrangement potential are characteristic of the transposable controlling elements discovered by McClintock.