An altered subunit configuration associated with the actively transcribed DNA of integrated adenovirus genes

The sensitivity to deoxyribonuclease I (DNAase I) of integrated adenovirus genes that encode mRNA has been compared to the sensitivity of adjacent viral DNA sequences that are not expressed as mRNA in two lines of adenovirus type 5-transformed hamster cells. We determined the concentrations of integrated DNA sequences homologous to different regions of the viral genome before and after mild DNAase I digestion of intact nuclei by measuring the rate of reassociation of restriction endonuclease fragments of labeled adenovirus DNA in the presence of DNA isolated from untreated and digested transformed cell nuclei. The HT14A cell line contains 2.4 copies of the left-hand 35% of the adenovirus type 5 genome per diploid quantity of cell DNA. Integrated sequences that are preferentially sensitive to DNAase I include all those encoding mRNA and some additional sequences, equivalent to 3-4 nucleosomes, to the 5′ side of the stable mRNA transcript. These results indicate that there is a striking correspondence, to within 3-4 nucleosomes, between the structure of an active transcription unit and the stable mRNA sequences encoded by it. The second cell line examined, HT14B, contains 5.5 copies of the left-hand 40% of the adenovirus type 5 genome per diploid quantity of cell DNA. Of the sequences complementary to viral mRNA, only half are preferentially sensitive to DNAase I. By contrast, the majority of the integrated viral DNA sequences that are not expressed as mRNA are resistant to DNAase I digestion in both this and the HT14A cell line. The results observed with HT14B cells suggest that only half of the integrated adenovirus sequences are in a chromatin conformation that permits transcription to occur.