Discriminating G-quadruplex topologies by bright-field microscopy

G-quadruplexes (G4s) are important regulatory elements for both genetics and epigenetics. Their biological functions mostly rely on their shapes, as a guanine (G)-rich sequence can fold into G4 structures in more than 20 different ways. This shape governs not only their folding kinetics and stability, but also the way G4s interact with cellular effectors, mostly G4-binding proteins. However, there are still some limitations in the simple and reliable identification of G4 topologies. Herein, we propose a novel method relying on the fast formation of G4 topology-dependent particles upon G4 interaction with colistin (COL): monitoring the number, size and shape of these aggregates using a routine bright-field microscope allows for profiling G4 topology in a reliable, inexpensive and user-friendly manner.