Kinetic Studies On Folding Of G-quadruplex DNA And RecQ5?-catalyzed Different DNA Unwinding

G-quadruplexes are special structures existing at the ends of human telomeres,the folding kinetics of which are essential for their functions,such as in the maintenance of genome stability and the protection of chromosome ends.In this work,we investigated the folding kinetics of G-quadruplex in different monovalent cation environments and determined the detailed kinetic parameters for Na+-and K+-induced G-quadruplex folding and for its structural transition from the basket-type Na+ form to the hybrid-type K+ form.More interestingly,although Li+ was often used as a control ion in previous studies of G-quadruplex folding with the synergistic effect of Li+ and Na+/K+ on G-quadruplex folding being not considered,we have found that Li+ can actually influence the folding kinetics of both Na+-and K+-induced G-quadruplexes significantly and in different ways,by changing the folding fraction of Na+-induced G-quadruplexes and greatly increasing the folding rates of K+-induced G-quadruplexes.The present work may shed new light on the roles of monovalent cations in G-quadruplex folding and should be useful for further studies of the underlying folding mechanism.RecQ5? is an essential DNA helicase in humans,playing important roles in DNA replication,repair,recombination and transcription.The unwinding activity and substrate specificity of RecQ5? is still elusive.Here,we used stopped-flow kinetic method to measure the unwinding and dissociation kinetics of RecQ5? with several kinds of DNA substrates,and found that RecQ5? could well unwind ss/dsDNA,forked DNA and Holiday junction,but was compromised in unwinding blunt DNA and G-quadruplex.Rec5? has the preferred unwinding specificity for certain DNA substrates containing the junction point,which may improve the binding affinity and unwinding activity of RecQ5?.Moreover,from a comparison with the truncated RecQ5?1-467,we discovered that the C-terminal domain might strongly influence the unwinding activity and binding affinity of RecQ5?.These results may shed light on the physiological functions and working mechanisms of RecQ5? helicase.