Functional studies of multiple thioredoxins from Mycobacterium tuberculosis

Abstract

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 trxA_Mtb 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.