CHARACTERIZATION OF THE KATG AND INHA GENES OF ISONIAZID-RESISTANT CLINICAL ISOLATES OF MYCOBACTERIUM-TUBERCULOSIS

Resistance to isoniazid in Mycobacterium tuberculosis has been associated with mutations in genes encoding the mycobacterial catalase-peroxidase (katG) and the InhA protein (inhA), Among the 26 isoniazid-resistant clinical isolates evaluated in this study, mutations in putative inhA regulatory sequences were identified in 2 catalase-positive isolates, katG gene alterations were detected in 20 strains, and 4 isolates had wild-type katG and inhA genes, Mutations in the katG gene were detected in all 11 catalase negative isolates: one frameshift insertion, two partial gene deletions, and nine different missense mutations were identified, An arginine-to-leucine substitution at position 463 was detected in nine catalase-positive isolates, However, site-directed mutagenesis experiments demonstrated that the presence of a leucine at codon 463 did not alter the activity of the M. tuberculosis catalase-peroxidase and did not affect the capacity of this enzyme to restore isoniazid susceptibility to isoniazid-resistant, KatG-defective Mycobacterium smegmatis pi-ii cells, These studies further support the association between katG and inhA gene mutations and isoniazid resistance in M. tuberculosis, while also suggesting that other undefined mechanisms of isoniazid resistance exist.