The Mycobacterium tuberculosis iniA gene is essential for activity of an efflux pump that confers drug tolerance to both isoniazid and ethambutol

Speaker: 張力云                           Time: PM2:00-3:00; 4/13/2005

Commentator: 劉清泉老師                  Place: 601

Abstract

The cell wall of Mycobacterium tuberculosis is the target for some antibiotics. Agents such as isoniazid (INH) and ethambutol (EMB) that act by inhibiting cell wall biosynthesis are the most effective antibiotics against this pathogen¹. Previous studies have found that a three-gene operon (iniBAC) expression was induced by treating M. tuberculosis with INH and EMB². In this study, the authors overexpressed M. tuberculosis iniA in M. bovis BCG 〔BCG (pMV261::iniA)〕by introducing a recombinant plasmid, pMV261::iniA, into this organism, and found that it caused increased viability of the organism upon exposure to INH. On the other hand, disruption of the iniA gene in M. tuberculosis H37Rv 〔H37Rv (DiniA::hyg)〕by phage-mediated allelic exchange resulted in increased susceptibility to INH. In addition, the level of ethidium bromide accumulation was higher in H37Rv (DiniA::hyg) compared to the parental strain. Treating BCG (pMV261::iniA) with reserpine that specifically inhibits the pump activity abolished the growth advantage in the presence of INH and the bacteria became susceptible to ethidium bromide. Determination of the 2-D crystalline structure of the IniA protein revealed that it formed hexamer structure that contained a central pore. These findings indicated that IniA may function through an MDR pump-like mechanism. However, there was no difference in INH accumulation between BCG (pMV261::iniA), BCG (pMV261), H37Rv (DiniA::hyg) and H37Rv. Therefore, it appeared that this pump may not pump INH directly, rather, IniA may form a part of a transporter complex and play a role in drug tolerance via an as yet unknown mechanism.

References:

1. Besra G. S., and Brennan P. J. The mycobacterial cell wall: biosynthesis of arabinogalactan and lipoarabinomannan. Biochem Soc Trans 25: 845–850 (1997).

2. Alland D., et al. Characterization of the Mycobacterium tuberculosis iniBAC promoter, a promoter that responds to cell wall biosynthesis inhibition. J Bacteriol 182: 1802–1811 (2000).

3. Colangeli R., et al. The Mycobacterium tuberculosis iniA gene is essential for activity of an efflux pump that confers drug tolerance to both isoniazid and ethambutol. Mol. Microbiology. 55 (6): 1829-1840 (2005).