Mechanistic and Phenotypic Characterisation of Rgg/SHP Quorum Sensing System in Streptococcus pneumoniae

The Rgg regulators with their short hydrophobic signalling peptides (SHPs) form part of a quorum sensing system (QS) in Streptococci. They play an important role in stress response, sugar metabolism, and virulence. Therefore, blocking phenotypic manifestations of Rgg/SHP QS system would be an effective strategy to abrogate streptococcal virulence. In this study, I focused on Rgg/SHP144 quorum sensing system in the important human pathogen Streptococcus pneumoniae by evaluating the functional importance of SHP144 residues towards the transcriptional activation of the system and Rgg144 binding. This information would allow in depth understanding of the system’s operation, and will be useful for developing anti-infectives that target Rgg/SHP144 system. The results showed that most of selected SHP144 residues are required for shp144 transcriptional activation, and residues at position I20 and P21 are critically important for mannose utilisation, capsule synthesis and oxidative stress resistance in vitro as well as for in vivo colonisation. Moreover, transcriptional activation of non-activating modified peptides mimics their binding capabilities, except that while SHP144-C13V17A and SHP144-C13P21A modifications abolished transcriptional activation of shp144 promoter, these modifications did not affect Rgg binding. SHP144-C13P21A modified peptide could competitively inhibit Rgg144 activation and decrease shp144 induction in a dose-dependent and sequence-specific manner. This modified peptide has also the capacity to diminish pneumococcal growth on mannose and render pneumococci susceptible to oxidative stress. QS systems are found widely in bacteria, and they are suggested to be potential anti-infective targets. Thus, this study lays the ground for developing effective inhibitors in future and demonstrates the potential utility of QS systems as anti-infective targets.