Functional Studies of Multiple Thioredoxins from Mycobacterium tuberculosis

Cytoplasmic protein reduction via generalized thiol/disulfide exchange reactions and maintenance of cellular redox homeostasis is mediated by the thioredoxin superfamily of proteins. Here, we describe the characterization of the thioredoxin system from Mycobacterium tuberculosis, whose genome bears the potential to encode three putative thioredoxins from the open reading frames designated trxA(Mtb), trxB(Mtb), and trxC(Mtb). We show that all three thioredoxins, overproduced in Escherichia coli, are able to reduce insulin, a model substrate, in the presence of dithiothreitol. However, we observe that thioredoxin reductase is not capable of reducing TrxA(Mtb) in an NADPH-dependent manner, indicating that only TrxB(Mtb) and TrxC(Mtb) are the biologically active disulfide reductases. The absence of detectable mRNA transcripts of trxA(Mtb) observed when M. tuberculosis strain H37Rv was cultivated under different growth conditions suggests that trxAMtb expression may be cryptic. The measured redox potentials of TrxB(Mtb) and TrxC(Mtb) (-262 +/- 2 mV and -269 +/- 2 mV, respectively) render these proteins somewhat more oxidizing than E. coli thioredoxin 1 (TrxA). In E. coli strains lacking components of cytoplasmic protein reduction pathways, heterologous expression of the mycobacterial thioredoxins was able to effectively substitute for their function.