Isoniazid affects multiple components of the type II fatty acid synthase system of Mycobacterium tuberculosis

Genetic and biochemical evidence has implicated two different target enzyme s for isoniazid (INH) within the unique type II fatty acid synthase (FAS) s ystem involved in the production of mycolic acids. These two components are an enoyl acyl carrier protein (ACP) reductase, InhA, and a beta -ketoacyl- ACP synthase, KasA. We compared the consequences of INH treatment of Mycoba cterium tuberculosis (MTB) with two inhibitors having well-defined targets: triclosan (TRC), which inhibits InhA; and thiolactomycin (TLM), which inhi bits KasA. INH and TLM, but not TRC, upregulate the expression of an operon containing five FAS II components, including kasA and acpM. Although all t hree compounds inhibit mycolic acid synthesis, treatment with INH and TLM, but not with TRC, results in the accumulation of ACP-bound lipid precursors to mycolic acids that were 26 carbons long and fully saturated. TLM-resist ant mutants of MTB were more cross-resistant to INH than TRC-resistant muta nts. Overexpression of KasA conferred more resistance to TLM and INH than t o TRC. Overexpression of InhA conferred more resistance to TRC than to INH and TLM. Co-overexpression of both InhA and KasA resulted in strongly enhan ced levels of INH resistance, in addition to cross-resistance to both TLM a nd TRC. These results suggest that these components of the FAS II complex a re not independently regulated and that alterations in the expression level of InhA affect expression levels of KasA. Nonetheless, INH appeared to res emble TLM more closely in overall mode of action, and KasA levels appeared to be tightly correlated with INH sensitivity.