DNA primer extension assay

The DNA primer extension assay was performed as described previously (57). Briefly, TET-labeled primer (1 μM) was annealed to 1.25 μM oligonucleotides containing G4 or non-G4 (S. pombe ade6⁺) sequences. Extension of the annealed primer was performed at 37°C for 1 min in the presence of either 1% (v/v) DMSO and 100 mM KCl or 0.1 μM PhenDC3 and 100 mM KCl using 0.063 U of an exonuclease-deficient Klenow fragment of Escherichia coli DNA polymerase (Thermo Fisher Scientific). Reaction products were separated on 10% polyacrylamide gels containing 8 M urea, 25% formamide, and 1 × Tris/borate/ethylenediaminetetraacetic acid (TBE). Bands were visualized and quantified using a Typhoon Scanner 9400 (GE Healthcare) and the ImageQuant 5.2 software (GE Healthcare).