Redox stress is one of the major challenges faced by Mycobacterium tuberculosis during early infection and latency. The mechanism of sensing and adaptation to altered redox conditions is poorly understood. whiB family of Mtb is emerging as an important class of stress responsive genes. WhiB3/Rv3416 has been shown to be important for pathogenesis in animal model and was recently shown to co-ordinate a Fe-S cluster. Here, we report a simple, rapid and efficient matrix-assisted refolding method and important redox properties of WhiB3. Similar to other WhiB proteins, WhiB3 also has four conserved cysteine residues, where two of them are present in a CXXC motif. The Fe-S cluster of WhiB3 remained bound in the presence of strong protein denaturant. Upon cluster removal due to oxidation, the four cysteine residues which are ligands of Fe-S cluster, formed two intra-molecular disulfide bridges where one of them is possibly between the cysteines of CXXC motif, an important feature of several thiol-disulfide oxido-reductases. Far-UV CD spectroscopy revealed the presence of both alpha-helices and beta-strands in apo WhiB3. The secondary structural elements of apo WhiB3 were found resistant for thermal denaturation. The results demonstrated that apo WhiB3 functions as a protein disulfide reductase similar to thioredoxins. The importance of WhiB3 in redox sensing and its possible role in mycobacterial physiology has been discussed. (C) 2008 Elsevier Inc. All rights reserved.