Identification of Nudix Hydrolase Family Members with an Antimutator Role in Mycobacterium tuberculosis and Mycobacterium smegmatis

Abstract

Mycobacterium tuberculosis and Mycobacterium smegmatis MutT1, MutT2, MutT3, and Rv3908 (MutT4) enzymes were screened for an antimutator role. Results indicate that both MutT1, in M. tuberculosis and M. smegmatis, and MutT4, in M. smegmatis, have that role. Furthermore, an 8-oxo-guanosine triphosphatase function for MutT1 and MutT2 is suggested.Oxidized guanine (8-oxo-G) is a potent mutagen because of its ambiguous pairing with cytosine and adenine. The Escherichia coli MutT protein specifically hydrolyzes both 8-oxo-deoxyguanosine triphosphate (8-oxo-dGTP) and 8-oxo-guanosine triphosphate (8-oxo-rGTP), preventing their misincorporation in DNA and RNA opposite template A (10,23,24,26). The MutT E. coli protein has an antimutator function, and it was the first enzyme of the MutT/Nudix hydrolase family, characterized by a 23-amino-acid region, to be studied. Nudix hydrolases, are so named because the ones characterized so far all hydrolyze a nucleoside diphosphate linked to some other moiety, X. Besides oxidized guanine, they were shown to degrade other substrates, such as NADH, GDP-mannose, or ADPribose (2,3,8,9,14,15,18).Here we have investigated the role of the putative Nudix hydrolases MutT1, MutT2, MutT3, and Rv3908 (MutT4) of M. tuberculosis (5) and their putative M. smegmatis homologues (sequences were obtained from The Institute for Genomic Research [TIGR] website; www.tigr.org) as antimutators (Fig. 1). Sequence subunit analysis did not suggest that these proteins were members of any of the known subfamilies of Nudix hydrolases (6, 13, 28). The mutT1 M. tuberculosis knockout mutant (MT1K) was isolated from the transposon library described previously (12). Selection was done by plating aliquots of each independent insertional mutant onto 7H10 plates containing rifampin (Rif) at 2 g/ml, two times the MIC of Rif for M. tuberculosis used by Morlock et al. (17). One of the clones giving a higher number of Rif-resistant (Rif r ) colonies than the wild-type strain harbored an insertion in mutT1. All other mutants were generated by allelic replacement using a replication temperature-sensitive vector harboring the counterselectable marker sacB to carry a kanamycin cassette-disrupted copy of the genes of interest (20). The bacterial strains, plasmids, and primers employed in this study are provided in the supplemental material. DNA isolation, cloning, and Southern hybridization were performed according to standard techniques. Complemented strains of the M. smegmatis mutants were generated by electroporation of the pVV16-derived vectors (11) described in the supplemental material. In 20 independent experiments, we carried out an adapted LuriaDelbruck fluctuation test, as described by Morlock et al. (17). The results obtained are summarized in Table 1. MutT1 deficiency in M. tuberculosis resulted in a 15.5-fold spontaneous mutation frequency increase by rifampin resistance screening compared with the wild-type strain. A similar 12-fold increase was observed for the mutT1 mutant of M. smegmatis. Furthermore, we observe...

Biochemical Properties of MutT2 Proteins from Mycobacterium tuberculosis and M. smegmatis and Their Contrasting Antimutator Roles in Escherichia coli

“…The enzymatic activity of the purified MtuMutT2 was assayed using different dNTPs and modified dNTPs, as described in Materials and Methods. As reported earlier (23,24), we observed that MtuMutT2 hydrolyzed 8-oxo-dGTP, 8-oxo-GTP, dCTP, and 5-Me-dCTP ( Fig. 2A, panels i to iv).…”

“…6). However, it may be noted that knockouts of MutT2 genes in M. tuberculosis and M. smegmatis resulted in an overall increase of similar mutation frequencies of about 1.5-and 1.7-fold, respectively (23). Although it should also …”

A distinct physiological role of MutY in mutation prevention in mycobacteria

“…Genome sequence analysis reports the presence of four probable MutT proteins in M. tuberculosis and another five Nudix-box-containing proteins to which MutT belongs. However, unlike the E. coli counterpart , none of the four MutTs shows a high specificity towards 8-oxo-dGTP (Dos Vultos et al, 2006). It has also been reported (Moreland et al, 2009) that the MutT2 of M. tuberculosis, which is the most similar to E. coli MutT, is actually a dCTPase.…”

Mycobacterium tuberculosis MutT1 (Rv2985) and ADPRase (Rv1700) Proteins Constitute a Two-stage Mechanism of 8-Oxo-dGTP and 8-Oxo-GTP Detoxification and Adenosine to Cytidine Mutation Avoidance

“…Therefore, it appears that MtuMutT2 may not be a major player in vivo, at least not as an 8-oxo-dGTPase. Genetic studies have also shown that the MutT2 deficiency in M. tuberculosis as well as in Mycobacterium smegmatis causes an insignificant increase of ϳ1.5-fold in mutation frequency, as assessed by appearance of Rif R colonies (28).…”

“…It should also be said that our observations do not rule out the possibility of some of the uncharacterized MutT (MtuMutT3, MtuMutT4) or other Nudix box proteins carrying out the function of efficiently hydrolyzing 8-oxodGTP and 8-oxo-GTP directly to their corresponding monophosphates. However, the fact that of the four MutT proteins described in mycobacteria, deficiency of MutT1 has the maximum mutator phenotype of approximately 15-fold in mycobacteria (28) suggests that MutT1 carries out the major MutT function. Thus, it appears that even in M. tuberculosis cells, MutT1 initiates the major function of detoxifying oxidatively damaged guanine nucleoside triphosphates.…”