Viruses as tools for studies on the molecular biology of mammalian cells

1. Multiple copies of intact adenovirus type 12 (Ad12) DNA are integrated into the DNA of Ad12-transformed hamster and Ad12-induced rat brain tumor cells. Free viral DNA is not present in the Ad12-transformed lines investigated. 2. Only few sites of integration are found in Al2-transformed hamster and Ad12-induced rat brain tumor cells. Integration may have occurred into repetitive cellular sequences at selective sites. These sites may be different in different cell lines, however, none of these sites has been analyzed in sufficient detail. 3. Three lines of Ad12-induced rat brain tumor cells exhibit identical patterns of integration. These lines have been derived from three brain tumors in one animal and may have evolved from the same transformed cell. 4. In Ad12-induced rat brain tumor cells early and late segments of the viral genome are expressed as polysome-associated messenger RNA. 5. In human cells productively infected with adenovirus type 2 (Ad2), a large number of viral genome copies are linked to cellular DNA early postinfection. There are only a few sites of recombination (illegitimate?) which probably lie in repetitive cellular DNA sequences. The functional significance of this frequent recombination is unknown.