FIGURE 1.

Circular dichroism spectra of single-stranded DNA with varying sequences. A, diagram of three 48-nt DNA templates investigated. hT48wt is comprised of eight hexameric repeats of a wild-type telomere DNA sequence. The hT48GG→CC construct is comprised of eight hexameric repeats of telomeric DNA in which the last two guanosines of each double repeat are mutated to cytidines. hT48→40ΔGG is comprised of eight hexameric repeats of telomeric DNA in which the last two guanosines of each double repeat are deleted. Each construct contains four consecutive POT1-TPP1 recognition sites, as indicated by the lines above each sequence. B, the CD spectra representing the average of three readings for each DNA sequence depicted in A are shown in buffer containing 75 mm NaCl. The hT48wt spectrum is characteristic of antiparallel G-quadruplexes typical of telomeric DNA in the presence of Na⁺. The spectra of hT48GG→CC and hT48→40ΔGG, however, are indicative of DNA adopting B-form helices, the most common ssDNA conformation. C, CD spectra as obtained in B but in the presence of 75 mm KCl. The hT48wt displays features that are characteristic of hybrid-type G-quadruplexes consisting of both parallel and antiparallel strands, known to be the dominant conformation of longer telomere sequences in the presence of K⁺. The spectra for both hT48GG→CC and hT48→40ΔGG lack these features, as both present as consistent with common B-form helices.