Mycobacterium tuberculosis, the causative agent of tuberculosis, is at increased 
risk of accumulating damaged guanine nucleotides such as 8-oxo-dGTP and 
8-oxo-GTP because of its residency in the oxidative environment of the host 
macrophages. By hydrolyzing the oxidized guanine nucleotides before their 
incorporation into nucleic acids, MutT proteins play a critical role in allowing 
organisms to avoid their deleterious effects. Mycobacteria possess several MutT 
proteins. Here, we purified recombinant M. tuberculosis MutT2 (MtuMutT2) and M. 
smegmatis MutT2 (MsmMutT2) proteins from M. tuberculosis (a slow grower) and M. 
smegmatis (fast growing model mycobacteria), respectively, for their biochemical 
characterization. Distinct from the Escherichia coli MutT, which hydrolyzes 
8-oxo-dGTP and 8-oxo-GTP, the mycobacterial proteins hydrolyze not only 
8-oxo-dGTP and 8-oxo-GTP but also dCTP and 5-methyl-dCTP. Determination of 
kinetic parameters (Km and Vmax) revealed that while MtuMutT2 hydrolyzes dCTP 
nearly four times better than it does 8-oxo-dGTP, MsmMutT2 hydrolyzes them 
nearly equally. Also, MsmMutT2 is about 14 times more efficient than MtuMutT2 in 
its catalytic activity of hydrolyzing 8-oxo-dGTP. Consistent with these 
observations, MsmMutT2 but not MtuMutT2 rescues E. coli for MutT deficiency by 
decreasing both the mutation frequency and A-to-C mutations (a hallmark of MutT 
deficiency). We discuss these findings in the context of the physiological 
significance of MutT proteins.