ETHAMBUTOL RESISTANCE IN MYCOBACTERIUM-TUBERCULOSIS - CRITICAL ROLE OF EMBB MUTATIONS

Ethambutol I(S,S')-2,2'-(ethylenediimino)di-1-butanol EMB], is a first -line drug used to treat tuberculosis. To gain insight into the molecu lar basis of EMB resistance, we characterized the 10-kb embCAB locus i n 16 EMB-resistant and 3 EMB-susceptible genetically distinct Mycobact erium tuberculosis strains from diverse localities by automated DNA se quencing and single-stranded conformation polymorphism analysis. All 1 9 organisms had virtually identical sequences for the entire 10-kb reg ion. Eight EMB-resistant organisms had mutations located in codon 306 of embB that resulted in the replacement of the wild-type Met residue with Ile or Val. Automated sequence analysis of the 5' region (1,892 b p) of embB in an additional 69 EMB-resistant and 30 EMB-susceptible M. tuberculosis isolates from diverse geographic localities and represen ting 70 distinct IS6110 fingerprints confirmed the unique association of substitutions in amino acid residue 306 of EmbB with EMB resistance . Six other embB nucleotide substitutions resulting in four amino acid replacements were uniquely found in resistant strains, Sixty-nine per cent of epidemiologically unassociated EMB-resistant organisms had an amino acid substitution not found in susceptible strains, and most (89 %) replacements occurred at amino acid residue 306 of EmbB, For strain s with the Met306Leu or Met306Val replacements EMB MICs were generally higher (40 mu g/ml) than those for organisms with Met306Ile substitut ions (20 mu g/ml). The data are consistent with the idea that amino ac id substitutions in EmbB alter the drug-protein interaction and thereb y cause EMB resistance.