DrRecQ regulates guanine quadruplex DNA structure dynamics and its impact on radioresistance in Deinococcus radiodurans

The GC-rich genome of Deinococcus radiodurans contains a very high density of putative guanine quadruplex (G4) DNA motifs and its RecQ (drRecQ) was earlier characterized as a 3 '-> 5 ' dsDNA helicase. We saw that N-Methyl mesoporphyrin IX (NMM), a G4 DNA binding drug affected normal growth as well as the gamma radiation resistance of the wild-type bacterium. Interestingly, NMM treatment and recQ deletion showed additive effect on normal growth but there was no effect of NMM on gamma radiation resistance of recQ mutant. The recombinant drRecQ showed ~400 times higher affinity to G4 DNA (K-d = 11.74 +/- 1.77 nM) as compared to dsDNA (K-d = 4.88 +/- 1.30 mu M). drRecQ showed ATP independent helicase function on G4 DNA, which was higher than ATP-dependent helicase activity on dsDNA. Unlike wild-type cells that sparingly stained for G4 structure with Thioflavin T (ThT), recQ mutant showed very high-density of ThT fluorescence foci on DNA indicating an important role of drRecQ in regulation of G4 DNA structure dynamics in vivo. These results together suggested that drRecQ is an ATP independent G4 DNA helicase that plays an important role in the regulation of G4 DNA structure dynamics and its impact on radioresistance in D. radiodurans.