The exceptional sensitivity of Mycobacterium tuberculosis to isonicotinic acid hydrazide (INH) lacks satis- factory definition. M. tuberculosis is a natural mutant in oxyR, a central regulator of peroxide stress response. The ahpC gene, which encodes a critical subunit of alkyl hydroperoxide re- ductase, is one of the targets usually controlled by oxyR in bacteria. Unlike in mycobacterial species less susceptible to INH, the expression of ahpC was below detection limits at the protein level in INH-sensitive M. tuberculosis and Mycobacte- rium bovis strains.
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