Organization of polyoma virus DNA in mouse tumor cell lines

Restriction mapping of polyoma virus DNA in mouse tumor cell lines gave patterns that varied with the cell line examined. These reflected differences in both the organization and the state of integration of virus genomes in the host chromosomes. (The cell lines were derived from tumors induced by polyoma virus in vivo and were propagated continuously in culture.) Two of the cell lines contained multiple copies of tandemly integrated virus genomes as well as free virus DNA molecules. Two other cell lines appeared to contain only integrated virus genomes arranged as tandem repeats. Based on restriction analysis with eleven different endonucleases, the virus DNA in one of the cell lines containing both free and integrated virus genomes was not detectably defective or hypermethylated. This is in contrast to most previously described polyoma virus transformed mouse cells. These virus genomes may, however, contain point mutations or unobserved rearrangements. The second cell line possessing free virus DNA molecules contained both nondefective and defective virus genomes. Most, if not all, defective virus genomes in this line were integrated. The two other cell lines possessing only detectable integrated virus DNA apparently contained only defective virus genomes. The defect in both cases was a small deletion (less than or equal to 0.2 kb) encompassing 0.12 map units on the physical map of polyoma virus DNA, a region coding for the proximal part of the large T antigen. Moreover, in contrast to the cell lines with free and detectably nondefective virus DNA, the virus DNA was extensively methylated in cell lines containing only integrated and defective virus genomes.