Formation of G-quadruplex and i-motif structures in the proximal promoter region of the RET oncogene Free

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A polypurine/polypyrimidine-rich sequence within the proximal promoter region of the human RET oncogene has been shown to be essential for RET basal transcription. Specifically, the polypurine-rich strand within this region consists of five consecutive runs of guanines, which is consistent with the general motif capable of forming an intramolecular G-quadruplex. Here, we demonstrate that in the presence of 100 mM K+, this sequence has the ability to adopt an intramolecular G-quadruplex structure in vitro. Moreover, comparative circular dichroism and DMS footprinting studies revealed the presence of a parallel-type intramolecular G-quadruplex structure containing three G-tetrads. The G-quadruplex-interactive agents, TMPyP4 and telomestatin, further stabilize the G-quadruplex structure. In addition, we demonstrate the complementary C-rich strand is capable of forming an i-motif structure in vitro as suggested by CD spectroscopy and chemical footprinting. Together, our results indicate that a G-quadruplex and i-motif structures can form within the proximal promoter region of the human RET oncogene, which could potentially play a role in the regulation of gene transcription.