Extension of G-quadruplex DNA by ciliate telomerase

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Abstract
Telomeric DNA can fold into four‐stranded structures known as G‐quadruplexes. Here we investigate the ability of G‐quadruplex DNA to serve as a substrate for recombinant Tetrahymena and native Euplotes telomerase. Inter‐ and intramolecular G‐quadruplexes were gel‐purified and their stability examined using native gel electrophoresis, circular dichroism (CD) and thermal denaturation. While intermolecular G‐quadruplexes were highly stable, they were excellent substrates for both ciliate telomerases in primer extension assays. In contrast, intramolecular G‐quadruplexes formed in K+ exhibited biphasic unfolding and were not extended by ciliate telomerases. Na+‐stabilised intramolecular G‐quadruplexes were extended by telomerase owing to their rapid rate of dissociation. The Tetrahymena telomerase protein component bound to inter‐ but not intramolecular K+‐stabilised G‐quadruplexes. This study provides evidence that parallel intermolecular G‐quadruplexes can serve as substrates for telomerase in vitro , their extension being mediated through direct interactions between this higher‐order structure and telomerase.