Single-Molecule Conformational Analysis of G-Quadruplex Formation in the Promoter DNA Duplex of the Proto-Oncogene C-Kit

摘要

DNA is a dynamic molecule whose canonical double-helix conformation must be disrupted to assume alternative structural forms to fulfill certain functional roles in nature. Sequence motifs containing stretches of tandem Gs can form four-stranded structures called G-quadruplexes that comprise stacked tetrads of mutually hydrogen-bonded guanines. G-quadruplex motifs occur throughout the human genome, raising the possibility of associated function. The single-stranded G-rich overhang of telomeres in various species can form G-quadruplexes and their formation in cilliates is regulated by telomere binding proteins. Quadruplex motifs are prevalent in nontelomeric parts of the genome, particularly in gene promoters, where quadruplex formation must necessarily compete with the DNA duplex. This would appear to be a formidable challenge given the high thermal stability normally associated with G-C-rich duplex DNA. A particularly well-studied case is the S1 nuclease hypersensitive element within the promoter of the proto oncogene c-MYC. This region contains a sequence proposed to form a G-quadruplex, whose structure has been elucidated by NMR studies on the corresponding single-stranded DNA sequence. Chemical biology studies suggest that quadruplex formation within this element is coupled to transcriptional down regulation of c-MYC. There have been numerous reports on the incidence of quadruplex forming motifs in promoters, and it has been proposed that quadruplex formation may be functionally related to gene transcription.