Probing telomeric G-quadruplex DNA structures in cells with in vitro generated single-chain antibody fragments

Guanine-rich sequences have been shown to readily form parallel or antiparallel G-quadruplex DNA structures in vitro. All telomeric repeat sequences contain stretches of guanine residues that can form quadruplex structures. In order to demonstrate the occurrence of the quadruplex structure in vivo, we generated by ribosome display, scFv antibodies specific for quadruplex DNA structures formed by the telomeric sequence of the ciliate Stylonychia. The macronucleus of this hypotrichous ciliate contains 10(8) telomere-capped nanochromosomes and was stained with the antibody recognizing the antiparallel G-quadruplex DNA in indirect immuno-fluorescence assays. This antibody was also used as a specific probe to study the interaction of the telomere end-binding proteins with the G-quadruplex during different stages of the cell cycle.