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J. Bacteriol., 06 1996, 3641-3649, Vol 178, No. 12
S Dhandayuthapani, Y Zhang, MH Mudd and V Deretic
Mycobacterium tuberculosis is a natural mutant with inactivated oxidative
stress regulatory gene oxyR. This characteristic has been linked to the
exquisite sensitivity of M. tuberculosis to isonicotinic acid hydrazide
(INH). In the majority of mycobacteria tested, including M. tuberculosis,
oxyR is divergently transcribed from ahpC, a gene encoding a homolog of the
subunit of alkyl hydroperoxide reductase that carries out substrate
peroxide reduction. Here we compared ahpC expression in Mycobacterium
smegmatis, a mycobacterium less sensitive to INH, with that in two highly
INH sensitive species, M. tuberculosis and Mycobacterium aurum. The ahpC
gene of M. smegmatis was cloned and characterized, and the 5' ends of ahpC
mRNA were mapped by S1 nuclease protection analysis. M. smegmatis AhpC and
eight other polypeptides were inducible by exposure to H2O2 or organic
peroxides, as determined by metabolic labeling and Western blot
(immunoblot) analysis. In contrast, M. aurum displayed differential
induction of only one 18-kDa polypeptide when exposed to organic peroxides.
AhpC could not be detected in this organism by immunological means. AhpC
was also below detection levels in M. tuberculosis H37Rv. These
observations are consistent with the interpretation that ahpC expression
and INH sensitivity are inversely correlated in the mycobacterial species
tested. In further support of this conclusion, the presence of plasmid-
borne ahpC reduced M. smegmatis susceptibility to INH. Interestingly,
mutations in the intergenic region between oxyR and ahpC were identified
and increased ahpC expression observed in deltakatG M. tuberculosis and
Mycobacterium bovis INH(r) strains. We propose that mutations activating
ahpC expression may contribute to the emergence of INH(r) strains.
Copyright © 1996, American Society for Microbiology
Oxidative stress response and its role in sensitivity to isoniazid in mycobacteria: characterization and inducibility of ahpC by peroxides in Mycobacterium smegmatis and lack of expression in M. aurum and M. tuberculosis
Department of Microbiology, University of Texas Health Science Center at San Antonio 78284-7758, USA.
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