The distribution of guanine-cytosine (GC) pairs in the DNA of the highly oncogenic simian adenovirus type 7 (SA7) and the non-oncogenic human adenovirus type 6 (Ad6) has been studied by thermal denaturation and CsCl density-gradient centrifugation. The differential of the DNA thermal denaturation curves shows the presence of pronounced peaks which indicates uneven distribution of GC pairs along the DNA chains and the presence of regions with GC content from 30 to 74% in SA7 DNA and from 40 to 68% in Ad6 DNA. The DNA restriction fragments obtained by treatment with EcoRI, RamHI, SalI, BglII and HindIII were subjected to CsCl density-gradient centrifugation. GC content of the fragments ranged from 45 to 70% for SA7 DNA and from 43 to 61% for Ad6 DNA. The GC content of the extreme left-hand fragments, where the transforming gene(s) is located, was higher than the average for SA7 DNA and lower than the average for Ad6 DNA. The most GC-rich regions were localized in the centre of the genome. The GC content of the right-hand part of both viral genomes was lower than the average.
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