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The nuclease hypersensitivity element III1 (NHE III1) upstream of the P1 and P2 promoters of c-MYC controls 80-90% of the transcriptional activity of this gene. The purine-rich strand in this region can form a G-quadruplex structure that is a critical part of the silencer element for this promoter. We have demonstrated that this G-quadruplex structure can form a mixture of four biologically relevant parallel-loop isomers, which upon interaction with the cationic porphyrin TMPyP4 are converted to mixed parallel/antiparallel G-quadruplex structures.
Seenisamy et al. (Tue,) studied this question.