Unusual DNA Structures in the Adenovirus Genome

More than 80% (approximately 29 kilobase pairs) of the adenovirus serotype 2 genome was surveyed for the presence of unusual DNA conformations. Seven recombinant DNAs containing the largest HindIII fragments of AD2 DNA were analyzed for the presence of negative supercoil-dependent S1 nuclease-sensitive sites. Four plasmids each contained a specific site of S1 nuclease sensitivity whereas the other three showed no reaction. Further investigation was focused on a plasmid containing one of the positively reacting fragments (fragment C) which contained the major late promoter at coordinate 16.4 on the genome; three serotypes (Ad2, Ad7, Ad12) were studied. Fine mapping studies revealed the S1-sensitive sites to be a small region (approximately 6 base pairs) located at the TATA box of the major late promoter in all three cases. Other determinations (supercoil relaxation, T7 gene 3 product sensitivity, bromoacetaldehyde reactivity, anomalous gel mobility, the influence of negative superhelical density on nuclease sensitivity) led to the conclusion that the B-helix deformation was not due to a previously recognized DNA conformation (left-handed Z-DNA, cruciform, bent DNA), but may be accounted for by the homopurine X homopyrimidine nature of this region.