Excision of amplified viral DNA at palindromic sequences from the adenovirus type 12‐transformed hamster cell line T637.

In the DNA of the adenovirus type 12 (Ad12)‐transformed hamster cell line T637 approximately 20‐22 viral DNA molecules per cell are covalently linked to cellular DNA. Spontaneously arising morphological revertants of T637 cells have lost the bulk of the viral DNA. We have been able to mimic the excision event of viral DNA, as it occurs during reversion, by autoincubation of isolated nuclei from T637 cells. The same Ad12 DNA sequences, which had been deleted in morphological revertants, proved highly sensitive to endogenous nucleases in isolated nuclei of T637 cells. Viral DNA sequences, which persisted in the revertants, are resistant to endogenous nucleases in isolated T637 nuclei. All attempts to clone the nuclease‐sensitive sites of Ad12 DNA in cell line T637 have so far failed. After denaturation and renaturation of T637 DNA followed by treatment with S1 nuclease, large fold‐back structures of DNA have been found. These snap‐back structures were derived from precisely those viral DNA restriction fragments which were uncloneable. The fragments containing palindromic sequences were both highly sensitive to endogenous nucleases in isolated T637 nuclei and were absent from the DNA of all revertant cell lines. Moreover, the palindromic sequences are susceptible to the phage T4‐specific endonuclease VII which specifically attacks cruciform structures in DNA. The peculiar structures at the termini of integrated Ad12 DNA molecules are highly sensitive to endogenous nucleases in isolated nuclei. These nucleases may be related to the reversion event.