THE THIOREDOXIN REDUCTASE SYSTEM OF MYCOPLASMAS

Representative species of the Mollicutes possess a thioredoxin reducta se system (NTS) composed of a low-molecular-mass thioredoxin (TRX) and NADPH-binding thioredoxin reductase (NTR), The TRXs of Mycoplasma pne umoniae and M. capricolum have molecular masses of 11.2 and 12 kDa, re spectively, and are stable at 90 degrees C for 10 min, Both TRXs react ed with monospecific polyclonal antibodies generated against the Bacil lus subtilis TRX, but not with anti-Escherichia coli TRX antisera. The M. capricolum and M. pneumoniae NTRs were partially purified and were found to be active with the homologous TRX, but not with the TRX of B . subtilis or E. coli, The NTS activity had an optimal ph of 6.5-7.5 a nd was dependent on NADPH as an electron donor, a requirement which co uld not be fulfilled by NADH, The genes encoding the TRX and NTR (trxA and trxB) of M. pneumoniae were cloned and sequenced, The comparative analysis of the predicted amino acid sequence of trxA showed that the 11.2 kDa protein (102 aa) shared 26-68% sequence similarity with prod ucts of other known trxA genes and contained the conserved active site Cys-Gly-Pro-Cys, The predicted amino acid sequence of trxB contained 315 residues with a conserved NADPH binding domain and FAD binding dom ains I and Il, The cysteine dithiol redox active region had isoleucine rather than threonine at the active site, as compared with other NTRs , The high activity of the NTS in mycoplasmas suggests that mycoplasma s may have evolved the NTS to protect themselves from the consequences of their self-generated oxidative challenge.