Coding capacity of a 35 percent fragment of the polyoma virus genome is sufficient to initiate and maintain cellular transformation.

Rat-1 cells were transfected with the restriction enzyme fragment of polyoma virus DNA that extends clockwise from the Bcl I site ((65.4 map units) to the EcoRI site (0/100 map units). Six transformed cell lines were obtained and one of them (BE-1) has been investigated in detail. The viral DNA that is integrated into host DNA in this line appeared to consist of two fragments arranged in a "head-to-tail" tandem with no detectable intervening host sequences. BE-1 cells contained polyoma virus small and middle tumor antigens that were indistinguishable from the corresponding tumor antigens from lytically infected cells. No large tumor antigen was detected but a "new" Mr 34,000 protein, which proved to be a truncated version of large tumor antigen, was immunoprecipitated by anti-tumor-antigen antiserum. After injection of 10(6) BE-1 cells into young syngeneic Fischer rats, tumors appeared within 3--4 weeks. Thus, the coding capacity of the Bcl I/EcoRI fragment of polyoma virus DNA is sufficient to enable the cells to produce all of small and middle tumor antigens and about a third of large tumor antigen, to transform cells stably in culture, and to produce tumors in vivo.