Effects of conserved residues and naturally occurring mutations on Mycobacterium tuberculosis RecG helicase activity

RecG is a helicase that is conserved in nearly all bacterial species. The prototypical Escherichia coli RecG promotes regression of stalled replication forks, participates in DNA recombination and DNA repair, and prevents aberrant replication. Mycobacterium tuberculosis RecG (RecGMtb) is a DNA-dependent ATPase that unwinds a variety of DNA substrates, although its preferred substrate is a Holliday junction. Here, we performed site-directed mutagenesis of selected residues in the wedge domain and motifs Q, I, Ib and VI of RecGMtb. Three of the 10 substitution mutations engineered were detected previously as naturally occurring SNPs in the gene encoding RecGMtb. Alanine substitution mutations at residues Q292, F286, K321 and R627 abolished the RecGMtb unwinding activity, whilst RecGMtb F99A, P285S and T408A mutants exhibited ~25-50 % lower unwinding activity than WT. We also found that RecGMtb bound ATP in the absence of a DNA cofactor.