The pyrimidine/purine-biased region located upstream of the EGF (epidermal growth factor) receptor gene transcription initiation sites was sensitive to S1 nuclease when under superhelical tension. The structural basis of this specific reactivity to S1 nuclease was probed by the use of diethyl pyrocarbonate. The patterns of modification suggested that the H-form proposed by Mirkin, Lyamichev, Drushlyak, Dobrynin, Fillippov and Frank-Kamenetskii [Nature (London) (1987) 330, 495-497], which includes an intramolecular triplex and a single-stranded region, was the most plausible model for the sequence tested. The results of dimethyl sulphate modification also supported this model.
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