Signal recognition particle RNA is critical for genetic competence and virulence of Streptococcus pneumoniae

Streptococcus pneumoniae (pneumococcus) causes a wide range of important human infectious diseases, including pneumonia, pneumonia-derived sepsis, otitis media, and meningitis. Pneumococcus produces numerous secreted proteins that are critical for normal physiology and pathogenesis. The membrane targeting and translocation of these secreted proteins are partly mediated by the signal recognition particle (SRP) complex, which consists of 4.5S small cytoplasmic RNA (ScRNA), and the Ffh, and FtsY proteins. Here, we report that pneumococcal ∆scRNA, ∆ffh, and ∆ftsY mutants were significantly impaired in competence induction, competence pili production, exogenous DNA uptake, and genetic transformation. Also, the ∆scRNA mutant was significantly attenuated in the mouse models of bacteremia and pneumonia. Interestingly, unlike the ∆scRNA, both ∆ffh and ∆ftsY mutants had growth defects on Todd-Hewitt Agar, which were alleviated by the provision of free amino acids or serum. Differences in nutritional requirements between ∆ffh and ∆ftsY vs ∆scRNA suggest that Ffh and FtsY may be partially functional in the absence of ScRNA. Finally, the insertase YidC2, which could functionally rescue some SRP mutations in other streptococcal species, was not essential for pneumococcal genetic transformation. Collectively, these results indicate that ScRNA is crucial for the successful development of genetic competence and virulence in pneumococcus.

Importance: Streptococcus pneumoniae (pneumococcus) causes multiple important infectious diseases in humans. The signal recognition particle (SRP) complex, which comprised 4.5S small cytoplasmic RNA (ScRNA), and the Ffh and FtsY proteins, mediates membrane targeting and translocation of secreted proteins in all organisms. However, the role of SRP and ScRNA has not been characterized during the induction of the competence system for genetic transformation and virulence in pneumococcus. By using a combination of genetic, biochemical, proteomic, and imaging approaches, we demonstrated that the SRP complex plays a significant role in membrane targeting of competence system-regulated effectors important for genetic transformation, virulence during bacteremia and pneumonia infections, and nutritional acquisition.