Intramolecular folding of a fragment of the cytosine-rich strand of telomeric DNA into an I-motif

In the recently discovered i-motif, four stretches of cytosine form two parallel-stranded duplexes whose C-C⁺ base pairs are fully intercalated. The i-motif may be recognized by characteristic Overhauser cross-peaks of the proton NMR spectrum, reflecting short H1′-H1′ distances across the minor groove, and short internucleotide amino-proton-H2'/H2′ across the major groove. We report the observation of such cross-peaks in the spectra of a fragment of the C-rich telomeric strand of vertebrates, d[CCCTAA]₃CCC. The spectra also demonstrate that the cytosines are base-paired and that proton exchange is very slow, as reported previously for the i-motif. From UV absorbance and gel chromatography measurements, we assign these properties to an i-motif which includes all or nearly all the cytosines, and which is formed by intramolecular folding at slightly acid or neutral pH. A fragment of telomeric DNA of Tetrahymena, d[CCCCAA]₃CCCC, has the same properties. Hence four consecutive C stretches of a C-rich telomeric strand can fold into an i-motif. Hypothetically, this could occur in vivo.