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 tw o parallel-stranded duplexes whose C.C+ base pairs are fully intercala ted. 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-pe aks in the spectra of a fragment of the C-rich telomeric strand of ver tebrates, d[CCCTAA]3CCC. The spectra also demonstrate that the cytosin es 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 o f Tetrahymena, d[CCCCAA]3CCCC, has the same properties. Hence four con secutive C stretches of a C-rich telomeric strand can fold into an i-m otif. Hypothetically, this could occur in vivo.