G-Quadruplex Hinders Translocation of BLM Helicase on DNA: A Real-Time Fluorescence Spectroscopic Unwinding Study and Comparison with Duplex Substrates

Sequences with the potential to form G-quadruplex structures are spread throughout genomic DNA. G-quadruplexes in promoter regions can play regulatory roles in gene expression. Expression of protein-encoding genes involves processing of DNA and RNA molecules at the level of transcription and translation, respectively. In order to examine how the G-quadruplex affects processing of nucleic acids, we established a real-time fluorescent assay and studied the unwinding of intramolecular G-quadruplex formed by the human telomere, ILPR and PSMA4 sequences by the BLM helicase. Through comparison with their corresponding duplex substrates, we found that the unwinding of intramolecular G-quadruplex structures was much less efficient than that of the duplexes. This result is in contrast to previous reports that multistranded intermolecular G-quadruplexes are far better substrates for the BLM and other RecQ family helicases. In addition, the unwinding efficiency varied significantly among the G-quadruplex structures, which correlated with the stability of the structures. These facts suggest that G-quadruplex has the capability to modulate the processing of DNA and RNA molecules in a stability-dependent manner and, as a consequence, may provide a mechanism to play regulatory roles in events such as gene expression.