ABSTRACT
Guanine rich regions of oligonucleotides fold into quadruple-stranded structures called G-quadruplexes (G4). Increasing evidence suggests that these G4 structures form in vivo with a crucial role in cellular processes, however, their direct observation in live cells remains a challenge. Here we unequivocally demonstrate that a fluorescent probe (DAOTA-M2) in conjunction with Fluorescence Lifetime Imaging Microscopy (FLIM) can identify G4 within nuclei of live and fixed cells. We have developed a new FLIM-based cellular assay to study the interaction of non-fluorescent small molecules with G4, which can be applied to a wide range of drug candidates. We demonstrate that DAOTA-M2 can be used to study G4 stability in live cells. Disruption of FancJ DNA helicase activity increases G4 lifetime, directly establishing for the first time its biological activity in mammalian cells.