9.B.282. The Isoniazid-resistance (IniABC) Family Isoniazid-induced genes, iniA, iniB, and iniC, were identified. IniB is homologous to cell wall proteins, and IniA contains a phosphopantetheine attachment site motif, suggestive of an acyl carrier protein. IniA is also induced by the antibiotic ethambutol, an agent that inhibits cell wall biosynthesis by a mechanism that is distinct from isoniazid (Alland et al. 1998; Ramaswamy et al. 2000). The operon responds to cell wall biosynthesis inhibition (Alland et al. 2000). Both IniA and IniB are essential for ethambutol resistance (Ahmad et al. 2004). Colangeli et al. 2005 showed that IniA is essential for the activity of a drug efflux system. Overexpression of iniA conferred resistance to ethidium bromide, and the deletion of iniA in M. tuberculosis resulted in increased accumulation of intracellular ethidium bromide. The pump inhibitor reserpine reversed both tolerance to isoniazid and resistance to ethidium bromide. Thus, IniA functions through an MDR-pump like mechanism, although IniA does not appear to directly transport INH from the cell. Analysis of two-dimensional crystals of the IniA protein revealed that this predicted transmembrane protein forms multimeric structures containing a central pore, providing further evidence that IniA is a pump component (Colangeli et al. 2005). Mutations in the iniA, iniB and iniC genes gave rise to isoniazid-sensitivity (Zhang et al. 2005). The operon is repressed by the Lsr2 histone-like AT-rich DNA binding protein that also represses the efpA efflux protein gene. EfpA is a drug resistance protein of the MFS (TC# 2.A.1.3.78), so the iniABC operon could be acting through this system (Colangeli et al. 2007).
References:
Ahmad, S., E. Mokaddas, and A.A. Jaber. (2004). Rapid detection of ethambutol-resistant Mycobacterium tuberculosis strains by PCR-RFLP targeting embB codons 306 and 497 and iniA codon 501 mutations. Mol. Cell Probes 18: 299-306.
Alland, D., A.J. Steyn, T. Weisbrod, K. Aldrich, and W.R. Jacobs, Jr. (2000). Characterization of the Mycobacterium tuberculosis iniBAC promoter, a promoter that responds to cell wall biosynthesis inhibition. J. Bacteriol. 182: 1802-1811.
Alland, D., I. Kramnik, T.R. Weisbrod, L. Otsubo, R. Cerny, L.P. Miller, W.R. Jacobs, Jr, and B.R. Bloom. (1998). Identification of differentially expressed mRNA in prokaryotic organisms by customized amplification libraries (DECAL): the effect of isoniazid on gene expression in Mycobacterium tuberculosis. Proc. Natl. Acad. Sci. USA 95: 13227-13232.
Colangeli, R., D. Helb, C. Vilchèze, M.H. Hazbón, C.G. Lee, H. Safi, B. Sayers, I. Sardone, M.B. Jones, R.D. Fleischmann, S.N. Peterson, W.R. Jacobs, Jr, and D. Alland. (2007). Transcriptional regulation of multi-drug tolerance and antibiotic-induced responses by the histone-like protein Lsr2 in M. tuberculosis. PLoS Pathog 3: e87.
Colangeli, R., D. Helb, S. Sridharan, J. Sun, M. Varma-Basil, M.H. Hazbón, R. Harbacheuski, N.J. Megjugorac, W.R. Jacobs, Jr, A. Holzenburg, J.C. Sacchettini, and D. Alland. (2005). The Mycobacterium tuberculosis iniA gene is essential for activity of an efflux pump that confers drug tolerance to both isoniazid and ethambutol. Mol. Microbiol. 55: 1829-1840.
Ramaswamy, S.V., A.G. Amin, S. Göksel, C.E. Stager, S.J. Dou, H. El Sahly, S.L. Moghazeh, B.N. Kreiswirth, and J.M. Musser. (2000). Molecular genetic analysis of nucleotide polymorphisms associated with ethambutol resistance in human isolates of Mycobacterium tuberculosis. Antimicrob. Agents Chemother. 44: 326-336.
Zhang, M., J. Yue, Y.P. Yang, H.M. Zhang, J.Q. Lei, R.L. Jin, X.L. Zhang, and H.H. Wang. (2005). Detection of mutations associated with isoniazid resistance in Mycobacterium tuberculosis isolates from China. J Clin Microbiol 43: 5477-5482.
Examples:
TC#	Name	Organismal Type	Example
9.B.282.1.1	Proteins of the iniBAC operon all give rise to multidrug resistance when mutated. They may function with the MFS drug efflux pump, EfpA (TC# 2.A.1.3.78). See family description for the evidence that ImiABC is involved in drug resistance (Colangeli et al. 2007). ImiA and ImiC have N-terminal regions that are homologous to each other. IniB is rich in glycines and shows apparent similarity to members of families 9.B.96, 1.B.89, 1.B.94 and 1.B.95, all of which may belong to the PE/PPE superfamily and are glycine-rich. The apparent similarity may be an artifactual due the presence of redundant sequences of similar limited amino acid compositions.		IniABC of Mycobacterium tuberculosis IniA, P9WJ99, 640 aas and 2 TMSs IniB, P9WJ97, 479 aas and 4 TMSs IniC, P9WJ95, 493 aas and 2 TMSs