The thioredoxin system in Streptococcus mutans and Streptococcus thermophilus: an insight on molecular and functional characterization of protein components

The Streptococcus genus includes several pathogenic species, such as S. mutans, the main responsible of dental caries, and the non pathogen S. thermophilus, used for the manufacture of dairy products. These fermenting bacteria are facultative anaerobes, as they tolerate moderate oxygen concentrations; however, Streptococci possess some crucial enzymes for the defence against ROS. The thioredoxin system is the key element for repairing cellulardamages caused by ROS, as it preserves the reduced state of cytosolic proteins. This work addresses the molecular and functional characterization of the thioredoxin components in S. mutans and S. thermophilus, to evaluate how this repairing system works in these sources. Usually, the thioredoxin system is composed by the flavoenzyme thioredoxin reductase (TrxB) and its natural substrate thioredoxin (TrxA) and uses NADPH as electron donor. The redundant putative genes encoding TrxB and TrxA in the S. mutans and S. thermophilus genome were analysed and the corresponding recombinant proteins were purified. In particular, a single TrxB was obtained from either S. mutans (SmTrxB) or S. thermophilus (StTrxB1), whereas two TrxA were prepared from either S. mutans (SmTrxA and SmTrxH1) or S. thermophilus (StTrxA1 and StTrxA2). The functionality of the recombinant enzymes was tested through specific biochemical assays. Indeed, both SmTrxB and StTrxB1 reduced the synthetic substrate DTNB in the presence of NADPH; conversely, among the four streptococcal thioredoxins, only SmTrxA and StTrxA1 accelerated the insulin reduction in the presence of DTT. The combined activity of the streptococcal thioredoxin components was tested through the insulin precipitation in the absence of DTT, in order to reconstitute the thioredoxin system. The assay is functioning with the combination of SmTrxB or StTrxB1 with either SmTrxA or StTrxA1. These results suggest that the streptococcal members of the thioredoxin system dispaly 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.