Streptococcus suis is a common part of the porcine respiratory microbiota. However, S. suis can also become pathogenic and cause life-threatening diseases in pigs as well as humans. A previous study identified the trpA gene (locus SSU1307) to be conditionally essential for in vivo survival by intrathecal infection of pigs with a transposon library of S. suis strain 10. In this work, we investigated the function of trpA encoding a putative tryptophan/tyrosine transport system substrate-binding protein. Therefore, we compared the growth capacities of the isogenic trpA-deficient mutant derivative (10∆trpA) with its parent strain.
Growth experiments in chemically defined media (CDM) revealed that growth of 10∆trpA depended on tryptophan concentration, suggesting that the mutant lacked the capability of high-affinity tryptophan binding and uptake. Complementation as well as the addition of a tryptophan-tripeptide to CDM with low tryptophan concentrations restored this growth defect.
Analyzing the operon structure of this transport system we demonstrated that trpA is co-transcribed with a putative permease and ATPase gene, respectively. Bioinformatics analysis identified a putative tryptophan T-box riboswitch in the 5’ untranslated region in front of this operon. Finally, qRT‑PCR and a reporter assay revealed trpA mRNA induction as well as reporter activation during growth under tryptophan-limited conditions.
In conclusion, our study showed that TrpA, probably functioning as a substrate-binding protein, is part of a putative tryptophan ABC transporter system regulated by a T-box riboswitch. Due to the tryptophan auxotrophy of S. suis, TrpA plays a crucial role for metabolic adaptation and growth of S. suis during infection.