Variation of quorum-sensing signal uptake systems

Quorum sensing (QS) is a bacterial cell-cell communication system which relies on the secretion and density-dependent group-wide detection of diffusible molecules. The NRPP family of QS systems is perhaps the most prevalent in Gram-positive bacteria. In this family, the signal molecule is a peptide which is expressed from a specific gene and undergo a series of cleavages during and after secretion. The mature signaling peptide is then transported into the cytoplasm through oligopeptide permeases to interact with a cytoplasmic receptor. Intriguingly, many Gram-positive species code for two, seemingly redundant, oligopeptide permease systems named Opp and App. Here, we explore the role of these two oligopermeases in a wild isolate of Bacillus subtilis. A phylogenetic analysis of the two systems show that while both are present in all B. subtilis strains, the Opp system is conserved through evolution, while the App system is horizontally transferred. To further explore the difference between the permease systems we analyzed their deletion mutants for their effect on biofilm formation and quorum-sensing response to a Rap-Phr system and to an exogenous NRPP system called PlcR-PapR. We show that Opp deletion reduces Surfactin production and represses biofilm formation whereas the App deletion had no effect. We further show that the App permease has higher affinity to the PapR signal peptide than the Opp, but a lower maximal transport rate. In contrast, the App system was unable to import the endogenous Phr signal we studied. These preliminary results start to elucidate the possible functional differences between the two transporters.