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Infection and Immunity, February 2003, p. 997-1000, Vol. 71, No. 2
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.2.997-1000.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
DNA Alkylation Damage as a Sensor of Nitrosative Stress in Mycobacterium tuberculosis
Steven I. Durbach,1 Burkhard Springer,2 Edith E. Machowski,1 Robert J. North,3 K. G. Papavinasasundaram,4 M. Jo Colston,4 Erik C. Böttger,2 and Valerie Mizrahi1*
MRC/NHLS/WITS Molecular Mycobacteriology Research Unit, School of Pathology, University of the Witwatersrand, and National Health Laboratory Service, Johannesburg, South Africa,1
Institute für Medizinische Mikrobiologie, Universität Zürich, Zürich, Switzerland,2
Trudeau Medical Institute, Saranac Lake, New York,3
Division of Mycobacterial Research, National Institute for Medical Research, The Ridgeway, Mill Hill, London, United Kingdom4
Received 17 December 2001/
Returned for modification 11 February 2002/
Accepted 31 October 2002

ABSTRACT
One of the cellular consequences of nitrosative stress is alkylation
damage to DNA. To assess whether nitrosative stress is registered
on the genome of
Mycobacterium tuberculosis, mutants lacking
an alkylation damage repair and reversal operon were constructed.
Although hypersensitive to the genotoxic effects of
N-methyl-
N'-nitro-
N-nitrosoguanidine
in vitro, the mutants displayed no phenotype in vivo, suggesting
that permeation of nitrosative stress to the level of cytotoxic
DNA damage is restricted.

TEXT
During the course of an infection,
Mycobacterium tuberculosis is expected to sustain significant levels of nitrosative stress.
Nitrate reductase activity provides endogenous nitrosative stress
through the production of · NO (
33). Although expressed
under aerobic and anaerobic conditions, this activity is required
at elevated levels during metabolic adaptation of the organism
to microaerophilia (
31) and may serve a respiratory function
within the burgeoning granuloma (
32). Exogenous nitrosative
stress increases significantly after the onset of acquired immunity
via the production of · NO by activated macrophages
(
4). The reaction of · NO with O
2 produces nitrous anhydride
(
3), which nitrosates amines and amides to produce compounds
that are metabolically activated to form potent DNA alkylating
agents (
10,
13,
15,
30). As such, alkylation damage of DNA can
be considered a downstream sensor of the level of nitrosative
stress to which a cell is exposed (Fig.
1).
In order to assess the permeation of nitrosative stress to this
level in
M. tuberculosis, we set out to develop mutant strains
that would be defective in dealing with the consequences of
alkylation damage. Alkylation of DNA can form cytotoxic (N-alkylation)
or promutagenic (O-alkylation) lesions (
30,
34). Since
M. tuberculosis contains an operon of genes predicted to be involved in the
repair (Rv1317c) and reversal (Rv1316c) of such lesions (
5,
9,
16), we constructed deletion mutants in this operon by two
independent strategies (Fig.
2) and investigated the growth
characteristics of the mutant strains in vitro and in vivo.
The SID-H and BS-SK strains both lacked the region of Adl that
contains the acceptor site (in AdaA) for methyl groups from
methyl phosphotriesters and regulates the adaptive response
(
24), as well as the entire AlkA domain. They also lacked the
region of Ogt containing the acceptor site for alkyl groups
from O-alkylated bases (
29).
The mutant strains grew normally in vitro (data not shown) but
were hypersensitive to the genotoxic effects of the alkylating
agent
N-methyl-
N'-nitro-
N-nitrosoguanidine (MNNG) (Fig.
3).
The viability of both the H37Rv and SID-H strains was reduced
in a dose-dependent manner by lower concentrations of MNNG,
but the mutant strain was ca. 100-fold more sensitive than the
wild type to the cytotoxic effects of this compound (Fig.
3A).
The BS-SK strain was found to be similarly hypersensitive to
the cytotoxic effects of MNNG (data not shown). The damage-hypersensitive
phenotype of strain SID-H was consistent with loss of the DNA
glycosylase (
alkA) component of
adl (
14) in the mutant strains.
This effect was possibly compounded by inactivation of the
adaA component of
adl, which would abrogate the ability of
M. tuberculosis to mount an adaptive response to alkylation damage (
24). However,
the functionality of this regulatory pathway has yet to be investigated
in
M. tuberculosis.
The mutagenic effects of MNNG were assessed from levels of induced
mutation to rifampin resistance in the various strains. Although
the average spontaneous mutation frequency of the SID-H mutant
strain in untreated controls was ca. twofold higher than that
of H37Rv, this difference was not statistically significant.
Both H37Rv and SID-H showed an increase in the frequency of
rifampin resistance following MNNG treatment, but the magnitude
of the increase was ca. 100-fold higher in SID-H over the concentration
range tested (Fig.
3B). This finding is consistent with the
functional inactivation of
ogt in the mutant strains, since
Rv1316c is the only recognizable gene for reversing promutagenic,
O-alkylation lesions (
22) in
M. tuberculosis (
16). Rifampin-resistant
mutants randomly selected from MNNG-treated and untreated control
cultures of the wild-type and mutant strains were genotyped
by PCR amplification and sequencing of the rifampin resistance-determining
region of the
rpoB gene (
21). The spectrum of
rpoB mutations
induced in SID-H closely matched those observed in rifampin-resistant
clinical isolates of
M. tuberculosis (
21) and in mutants selected
in vitro (
17) and was consistent with the exacerbation of transition
mutagenesis as the predicted consequence of persisting promutagenic
lesions (data not shown).
To determine whether the mutation in SID-H exerted polar effects on neighboring genes, a fragment carrying a wild-type copy of the adl-ogt operon and flanking sequences (Fig. 2) was integrated at the attB chromosomal locus of SID-H and the damage sensitivity and induced mutation frequency of the resulting complemented strain in response to treatment with 3 µM MNNG were assessed alongside those of SID-H. Comparison with the data of Fig. 3 revealed that genetic complementation of SID-H restored the damage sensitivity and the induced mutation frequency to wild-type levels (95% ± 36% survival of the treated culture versus the untreated control and an induced mutation frequency of 5.7 x10-7 ± 0.7 x10-7, respectively). This result confirmed that the phenotype of the SID-H mutant was solely attributable to the mutation in the adl-ogt operon.
To examine whether the alkylation damage repair and reversal defects of the
adl-ogt mutants would be manifested in a growth phenotype in vivo, C57BL/6 mice were infected with SID-H or H37Rv and bacillary loads in the lungs, liver, and spleen were measured at a relatively early stage of stationary infection, i.e., 30 days after the infection entered stationary phase in the lungs. However, no significant differences were observed in the organ counts from mice infected with the mutant strain compared to those of the wild-type control (Fig. 4). The BS-SK mutant displayed a similar phenotype in BALB/c mice: no significant differences in bacillary loads were observed between the mutant and wild-type strains at relatively early (days 30 and 55 postinfection) or later (day 104) stages of stationary infection (data not shown). The phenotype of the
adl-ogt mutant strains thus parallels that of a recA mutant of M. bovis BCG, which was hypersensitive to DNA damage in vitro but also displayed no growth impairment in mice (26).
The results of this study suggest that permeation of nitrosative
stress to the level of cytotoxic alkylation damage to the genome
of
M. tuberculosis is restricted in vivo, at least up to 15
weeks postinfection in the murine model. Restriction in the
levels of cytotoxic alkylation damage sustained in vivo might
be attributable to the action of defense or detoxification systems,
which protect against nitrosative stress (
2,
8,
11,
12,
19,
23). Alternatively, the environments encountered by
M. tuberculosis during the course of an infection may not favor alkylation damage;
specifically, at the times of bacillary load determination,
· NO may be preferentially sequestered by O
2 ·
-, which would increase the potential for oxidative (
6,
28)
rather than alkylative damage (Fig.
1). An assessment of the
growth phenotypes of mutant strains defective in the ability
to repair replication-blocking oxidative lesions would be revealing
in this respect. Finally, the alkylating agent(s) generated
in
M. tuberculosis might be of the promutagenic rather than
the cytotoxic type, which could lead to a mutator phenotype
without affecting the bacillary load. The extent of promutagenic
damage that may occur and its role in genomic diversification
and the evolution of drug resistance thus remain important areas
for future investigation.

ACKNOWLEDGMENTS
This work was supported by grants from the GlaxoSmithKline Action
TB Initiative, the Wellcome Trust (061017), the Medical Research
Council of South Africa, the South African Institute for Medical
Research, the National Research Foundation (V.M.), and the Deutsche
Forschungsgemeinschaft (BO 820/11-2 and BO 820/13-1 [E.C.B.]).
V.M. was also supported by an International Research Scholars
grant from the Howard Hughes Medical Institute.
We thank Helena Boshoff and Neil Stoker for helpful discussions.

FOOTNOTES
* Corresponding author. Mailing address: MRC/NHLS/WITS Molecular Mycobacteriology Research Unit, NHLS, P.O. Box 1038, Johannesburg 2000, South Africa. Phone: 2711-4899370. Fax: 2711-4899001. E-mail:
mizrahiv{at}pathology.wits.ac.za.

Editor: S. H. E. Kaufmann

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Infection and Immunity, February 2003, p. 997-1000, Vol. 71, No. 2
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.2.997-1000.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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