G-quadruplex forming structural motifs in the genome of Deinococcus radiodurans and their regulatory roles in promoter functions.

Deinococcus radiodurans displays compromised radioresistance in the presence of guanine quadruplex (G4)-binding drugs (G4 drugs). Genome-wide scanning showed islands of guanine runs (G-motif) in the upstream regions of coding sequences as well as in the structural regions of many genes, indicating a role for G4 DNA in the regulation of genome functions in this bacterium. G-motifs present upstream to some of the DNA damage-responsive genes like lexA, pprI, recF, recQ, mutL and radA were synthesized, and the formation of G4 DNA structures was probed in vitro. The G-motifs present at the 67th position upstream to recQ and at the 121st position upstream to mutL produced parallel and mixed G4 DNA structures, respectively. Expression of β-galactosidase under recQ and mutL promoters containing respective G-motifs was inhibited by G4 drugs under normal growth conditions in D. radiodurans. However, when such cells were exposed to γ radiation, mutL promoter activity was stimulated while recQ promoter activity was inhibited in the presence of G4 drugs. Deletion of the G-motif from the recQ promoter could relax it from G4 drug repression. D. radiodurans cells treated with G4 drug showed reduction in recQ expression and γ radiation resistance, indicating an involvement of G4 DNA in the radioresistance of this bacterium. These results suggest that G-motifs from D. radiodurans genome form different types of G4 DNA structures at least in vitro, and the recQ and mutL promoters seem to be differentially regulated at the levels of G4 DNA structures.