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J. Bacteriol. doi:10.1128/JB.00026-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Iron dependent RNA-binding activity of Mycobacterium tuberculosis aconitase

Laboratory of Molecular and Cellular Biology, Centre for DNA Fingerprinting and Diagnostics, Hyderabad, 500076, India; University of Hyderabad, Hyderabad 500046, India; Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur, Bangalore 560012, India

^* To whom correspondence should be addressed. Email: seh{at}ouhyd.ernet.in.

	Abstract

Cellular iron levels are closely monitored by iron regulatory and sensor proteins of Mycobacterium tuberculosis, for survival inside the macrophages. One such class of proteins systematically studied in eukaryotes and reported in few prokaryotes is Iron responsive proteins (IRPs). These IRPs bind to iron responsive elements (IREs) present at untranslated regions (UTRs) of mRNAs and are responsible for post-transcriptional regulation of expression of proteins involved in iron homeostasis. Amino acid sequence analysis of M. tuberculosis aconitase (Acn), a TCA cycle enzyme, showed the presence of the conserved residues of IRP class of proteins. We demonstrate that M. tuberculosis Acn is bifunctional. It is a monomeric protein that is enzymatically active in converting isocitrate to cis-aconitate at a broad pH range of 7-10 (optimum pH 8). M. tuberculosis Acn also behaves like an IRP by binding, as evident from gel retardation assays, to known mammalian IRE-like sequences and to predicted IRE-like sequences present at the 3' UTR of thioredoxin (trxC) and 5' UTR of iron dependent repressor and activator (ideR) of Mycobacterium tuberculosis. M. tuberculosis Acn when reactivated with Fe⁺⁺ functions as a TCA cycle enzyme, but upon iron depletion, by a specific iron chelator, behaves like an IRP, binding to the selected IREs in-vitro. Since iron is required for the aconitase activity and inhibits the RNA binding activity of the aconitase, both the activities of M. tuberculosis Acn are mutually exclusive. Our results demonstrate the bifunctional nature of M. tuberculosis Acn pointing to its likely role in iron homeostasis.