Properties of the thioredoxin system in microaerophiles from the Streptococcus genus

The thioredoxin system, involved in the preservation of the reduced state of citoplasmic proteins, is composed of two enzymes, thioredoxin (TrxA) and thioredoxin reductase (TrxB). To investigate on the properties of this system in the Streptococcus genus, the strains S. mutans, a pathogen involved in the development of dental caries, and S. thermophilus, a non pathogenic species employed in food industry, were selected. These fermenting facultative anaerobes are aerotolerant, as they possess superoxide dismutase, the first key enzyme involved in the control of reactive oxygen species; on the other hand, S. mutans and S. thermophilus lack catalase. The redundant putative genes encoding TrxA and TrxB in the genome of S. mutans and S. thermophilus were studied. Their predicted amino acid sequence was analysed to exclude products without the typical CXXC consensun. One TrxB gene in each strain, whereas two and three genes for TrxA remained in S. mutans and S. thermophilus , respectively. The TrxB gene from these strains was cloned and expressed, together with one TrxA gene from each strain. The activity of the recombinant enzymes was tested. Each TrxB catalysed the NADPH-dependent reduction of dithiobisnitrobenzoate, and each TrxA induced the insulin precipitation in the presence of dithiothreitol. When the combined activity of the homologous enzymes was tested in the presence of NADPH as electron donor and human insulin as the TrxA substrate, only the reconstitued S. thermophilus system was active, thus demonstrating the direct functional interaction between the two homologous components. For the reconstitution of the S. mutans system, the other putative TrxA genes from this strain will be analysed. The study of the molecular and biochemical properties of the recombinant enzymes is in progress.