A SINGLE-POINT MUTATION IN THE EMBB GENE IS RESPONSIBLE FOR RESISTANCE TO ETHAMBUTOL IN MYCOBACTERIUM-SMEGMATIS

Ethambutol [EMB; dextro-2,2'-(ethylenediimino)-di-1-butanol] is an eff ective drug when used in combination with isoniazid for the treatment of tuberculosis. It inhibits the polymerization of arabinan in the ara binogalactan and lipoarabinomannan of the mycobacterial cell wall. Rec ent studies have shown that arabinosyltransferases could be targets of EMB. These enzymes are encoded by the emb locus that was identified i n Mycobacterium smegmatis, Mycobacterium leprae, Mycobacterium avium, and Mycobacterium tuberculosis. We demonstrate that a missense mutatio n in the M. smegmatis embB gene, one of the genes of the emb locus, co nfers resistance to EMB. The level of resistance is not dependent on t he number of copies of the mutated embB gene, indicating that this is a true mechanism of resistance. The mutation is located in a region of the EmbB protein that is highly conserved among the different mycobac terial species. We also identified in this region two other independen t mutations that confer EMB resistance. Furthermore, mutations have re cently been described in the same region of the EmbB protein from clin ical EMB-resistant M. tuberculosis isolates. Together, these data stro ngly suggest that one of the mechanisms of resistance to EMB consists of missense mutations in a particular region of the EmbB protein that could be directly involved in the interaction with the EMB molecule.