The thioredoxin system in the dental caries pathogen Streptococcus mutans and the food-industry bacterium Streptococcus thermophilus.

Abstract The Streptococcus genus includes the pathogenic species Streptococcus mutans , the main responsible of dental caries, and the safe microorganism Streptococcus thermophilus , used for the manufacture of dairy products. These facultative anaerobes control the levels of reactive oxygen species (ROS) and indeed, both S. mutans and S. thermophilus possess a cambialistic superoxide dismutase, the key enzyme for a preventive action against ROS. To evaluate the properties of a crucial mechanism for repairing ROS damages, the molecular and functional characterization of the thioredoxin system in these streptococci was investigated. The putative genes encoding its protein components in S. mutans and S. thermophilus were analysed and the corresponding recombinant proteins were purified. A single thioredoxin reductase was obtained from either S. mutans ( Sm TrxB) or S. thermophilus ( St TrxB1), whereas two thioredoxins were prepared from either S. mutans ( Sm TrxA and Sm TrxH1) or S. thermophilus ( St TrxA1 and St TrxA2). Both Sm TrxB and St TrxB1 reduced the synthetic substrate DTNB in the presence of NADPH, whereas only Sm TrxA and St TrxA1 accelerated the insulin reduction in the presence of DTT. To reconstitute an in vitro streptococcal thioredoxin system, the combined activity of the thioredoxin components was tested through the insulin precipitation in the absence of DTT. The assay functions with a combination of Sm TrxB or St TrxB1 with either Sm TrxA or St TrxA1. These results suggest that the streptococcal members of the thioredoxin system display a direct functional interaction between them and that these protein components are interchangeable within the Streptococcus genus. In conclusion, our data prove the existence of a functioning thioredoxin system even in these microaerophiles.