Mce2R/Rv0586 of Mycobacterium tuberculosis is the functional homologue of FadRE.  coli. 
Lipid metabolism is critical to Mycobacterium tuberculosis survival and infection.  Unlike Escherichia coli, which has a single FadR, the M. tuberculosis genome encodes five proteins of the FadR sub-family.  While the role of E. coli FadR as a regulator of fatty acid metabolism is well known, the definitive functions of M. tuberculosis FadR proteins are still under investigation.  An interesting question about the M. tuberculosis FadRs remains open: which one of these proteins is the functional homologue of E. coli FadR?  To address this, we have applied two different approaches.  The first one was the bioinformatics approach and the second one was the classical molecular genetic approach involving complementation studies.  Surprisingly, the results of these two approaches did not agree.  Among the five M. tuberculosis FadRs, Rv0494 shared the highest sequence similarity with FadRE.  coli and Rv0586 was the second best match.  However, only Rv0586, but not Rv0494, could complement E. coli ∆fadR, indicating that Rv0586 is the M. tuberculosis functional homologue of FadRE.  coli.  Further studies showed that both regulators, Rv0494 and Rv0586, show similar responsiveness to LCFA, and have conserved critical residues for DNA binding.  However, analysis of the operator site indicated that the inter-palindromic distance required for DNA binding differs for the two regulators.  The differences in the binding site selection helped in the success of Rv0586 binding to fadB upstream over Rv0494 and may have played a critical role in complementing E. coli ∆fadR. Further, for the first time, we report the lipid-responsive nature of Rv0586.