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

Resistance to isoniazid in Mycobacterium tuberculosis has been associa ted with mutations in genes encoding the mycobacterial catalase-peroxi dase (katG) and the InhA protein (inhA), Among the 26 isoniazid-resist ant clinical isolates evaluated in this study, mutations in putative i nhA regulatory sequences were identified in 2 catalase-positive isolat es, katG gene alterations were detected in 20 strains, and 4 isolates had wild-type katG and inhA genes, Mutations in the katG gene were det ected 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 d etected in nine catalase-positive isolates, However, site-directed mut agenesis experiments demonstrated that the presence of a leucine at co don 463 did not alter the activity of the M. tuberculosis catalase-per oxidase and did not affect the capacity of this enzyme to restore ison iazid susceptibility to isoniazid-resistant, KatG-defective Mycobacter ium smegmatis pi-ii cells, These studies further support the associati on between katG and inhA gene mutations and isoniazid resistance in M. tuberculosis, while also suggesting that other undefined mechanisms o f isoniazid resistance exist.