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Journal of Bacteriology, April 2006, p. 3159-3161, Vol. 188, No. 8
0021-9193/06/$08.00+0     doi:10.1128/JB.188.8.3159-3161.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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

T. Dos Vultos,1 J. Blázquez,2 J. Rauzier,1 I. Matic,3 and B. Gicquel1*

Unité de Génétique Mycobactérienne, Institut Pasteur, Paris, France,1 Departamento de Biotecnología Microbiana, Centro Nacional de Biotecnología (CSIC), Madrid, Spain,2 INSERM U570, Faculte Necker-Enfants Malades, Universite Rene Descartes, Paris, France3

Received 18 November 2005/ Accepted 27 January 2006


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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.


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TEXT
 
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 ADP-ribose (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 µg/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 (Rifr) 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 Luria-Delbruck 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 observed a striking 48.1-fold increase in spontaneous Rifr colonies for the MutT4-deficient M. smegmatis strain. By contrast, we observed no increase for the MutT4-deficient M. tuberculosis strain. One may hypothesize the existence of enzymes with functions redundant to that of MutT4 in M. tuberculosis, thus masking the effect of MutT4 deficiency in these species. Defects in mutT2 and mutT3 genes resulted in no apparent differences between mutants and wild type. Moreover, similar results were obtained when screening for isoniazid resistance (data not shown). Complementation of the mutT1 and mutT4 mutants of M. smegmatis with wild-type copies of the M. smegmatis or M. tuberculosis mutT1 and mutT4 genes reduced the mutation frequency to that seen in the wild type (see the supplemental material), indicating that the genes from both sources were capable of restoring wild-type mutation frequencies in the mutants.


Figure 1
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  		FIG. 1. MutT protein sequence alignment. M. tuberculosis Rv2985 (MutT1), Rv1160 (MutT2), Rv0413 (MutT3), and Rv3908 (MutT4) and M. smegmatis MSMEG2389 (MutT1), MSMEG5134(MutT2), MSMEG0784 (MutT3), and MSMEG6883 (MutT4) were selected from the M. tuberculosis (Tuberculist) and M. smegmatis (TIGR) genomes because of their annotation or after a BLAST analysis. These sequences were compared to E. coli MutT by using alignments available at http://www.ch.embnet.org/software/ClustalW.html. The detected region of similarity is shown. *, absolutely conserved residues that define the mutT or Nudix motif; :, residues that are strongly conserved.


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  		TABLE 1. Frequency and nature of spontaneous rpoB mutations

In order to investigate the possible function of these mycobacterial MutT proteins, we sequenced the rpoB gene cluster I region of 32 randomly picked Rifr colonies derived from each mutant, as described by Rad et al. (22). The results are shown in Table 1. As described previously for MutT-defective E. coli (10), we observed 95- to 165- and 7- to 32-fold increases in A-to-C transversions for the mutT1- and mutT2-deficient M. tuberculosis and M. smegmatis, respectively, in comparison with the wild type. We found that M. smegmatis MutT3-deficient strains displayed 518-codon deletions and previously undescribed double 508/509-codon deletions (19). MutT4 deficiency in M. smegmatis was associated with a very high number of T-to-C mutations.

To assess the possible function of these proteins, enzyme assays were performed with 8-oxo-dGTP and other known Nudix hydrolase substrates. The strains for production of the recombinant proteins were obtained by transforming the pVV16-derived vectors in M. smegmatis mc2155. Recombinant proteins, which carry a six-histidine tag at the carboxyl terminus, were partially purified using a Ni-nitrilotriacetic acid superflow QIAGEN resin as described by Stadthagen et al. (25). The standard reaction mixture was in 50 µl of a solution containing 50 mM Tris-Cl (pH 8.5), 5 mM Mg2+, 25 mM NaCl, 2 mM substrate, 0.5 U of yeast inorganic pyrophosphatase for substrates such as (deoxy)nucleoside triphosphates and their derivatives (or 4 U of alkaline phosphatase for all other substrates), and the excess 5 µg of the partially purified extracts. The solution was incubated at 37°C for 30 min, and the reaction was stopped by the addition of 250 µl of 4 mM EDTA (or a Norit suspension to remove unreacted triphosphates). The liberated inorganic orthophosphate was assayed by the colorimetric procedure of Fiske and SubbaRow (7) as modified by Ames and Dubin (1). The results, normalized for a control reaction of an M. smegmatis strain carrying the empty vector, are shown in Fig. 2. As suggested by the rpoB sequencing, MutT1 and MutT2 displayed a clear 8-oxo-GTPase activity. Additionally, as reported for human and E. coli MutT, these proteins exhibited hydrolytic activity on dGTP. Although conclusions on the substrate specificities of our enzymes cannot readily be drawn from our experiments, MutT4 seemed to display a greater hydrolytic activity on dATP than on other substrates under the conditions used in our assay. No recombinant MutT3 protein could be obtained in a suitable form for analysis. Because no antimutator phenotype was suggested for this gene, we did not pursue its analysis.


Figure 2
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  		FIG. 2. Rate of hydrolysis of the various (deoxy)nucleoside triphosphate substrates for the partially purified extracts. Experiments were performed as described in the text.

For M. tuberculosis, with the exception of the dnaE2 role in inducible mutagenesis (4), no other gene was found to be associated with a mutator or antimutator phenotype (16). Strains of the M. tuberculosis W-Beijing family were linked with an increased risk of resistance (21). Sequencing studies revealed that W-Beijing strains could be divided into several branches according to the accumulation of unique missense alterations in three putative antimutator genes, including two of the oxidative damage-related mutT type, mutT2 and mutT4 (22). A previous study by Werngren and Hoffner (27) revealed no mutator phenotype for W-Beijing strains. However, those results do not specify which types of W-Beijing strains were used; hence no definite conclusions could yet be made. Here we describe an apparent antimutator role for the MutT1 enzyme of M. tuberculosis and M. smegmatis. Additionally, our results suggest that this enzyme shares with MutT2 the function of E. coli MutT, which is in apparent contrast with the observed antimutator role for MutT1, but not for MutT2, and this may account for the low number of Rifr colonies found for the mutator MutT1-deficient strains, compared with the numbers observed in other bacteria deficient in MutT enzymes. At present, we have no satisfactory explanation for the conflicting data. One may hypothesize that MutT1 has broader substrate specificity than MutT2. This is the first example of a gene with an antimutator role in M. tuberculosis. Besides 8-oxo-dGTPase activity, this gene's complete role is still unknown. Further studies might elucidate its predominance over MutT2.


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ACKNOWLEDGMENTS
 
This work received support from the European Commission (grant QLK2-CT-2000-00630 and VACSIS project ICA4-CT-2002-10052), the GPH-05 of Institut Pasteur, and the Louis D. French Award. J. Blázquez was supported by the grant BFU2004-0079 from the Spanish Ministerio de Educación y Ciencia.

We thank Mary Jackson and Jana Kurdulakova for help with the manuscript and critical discussions.


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FOOTNOTES
 
* Corresponding author. Mailing address: Unité de Génétique Mycobactérienne, Institut Pasteur, 28 rue du Dr Roux, 75015 Paris, France. Phone: 33 (1) 40 61 88 28. Fax: 33 (1) 45 68 88 43. E-mail: bgicquel{at}pasteur.fr. Back

{dagger} Supplemental material for this article may be found at http://jb.asm.org/. Back


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Journal of Bacteriology, April 2006, p. 3159-3161, Vol. 188, No. 8
0021-9193/06/$08.00+0     doi:10.1128/JB.188.8.3159-3161.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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