Secretion, maturation and activity of a quorum-sensing peptide (GSP) inducing bacteriocins transcription in Streptococcus gallolyticus

Abstract Streptococcus gallolyticus subsp. gallolyticus ( Sgg ) is an emerging opportunistic pathogen responsible for septicemia and endocarditis in the elderly. Invasive infections by Sgg are strongly linked to the occurrence of colorectal cancer (CRC). It was previously shown that increased secondary bile salts in CRC-conditions enhances the bactericidal activity of gallocin, a bacteriocin produced by Sgg , enabling it to colonize the mouse colon by outcompeting resident enterococci. In a separate study, we have shown that Sgg produces and secretes a 21-mer peptide that activates bacteriocin production. This peptide was named CSP because of its sequence similarity with competence stimulating peptides found in other streptococci. Here we demonstrate that CSP is a bona fide quorum-sensing peptide involved in activation of gallocin gene transcription. We therefore refer to CSP as GSP (gallocin stimulating peptide). GSP displays some unique features since its N -terminal amino-acid lies three residues after the double glycine leader sequence. Herein, we set out to investigate the processing and export pathway that leads to mature GSP. We also conducted the first comprehensive structure-activity relationship (SAR) of Sgg GSP to identify its key structural features. Significance Streptococcus gallolyticus subsp. gallolyticus ( Sgg ) is an opportunistic pathogen associated with colorectal cancer (CRC) and endocarditis. Sgg utilizes quorum-sensing (QS) to regulate the production of a bacteriocin (gallocin) and gain selective advantage in colonizing the colon. In this manuscript, we report 1) the first structure-activty relationship study of the Sgg QS pheromone that regulates gallocin production; 2) evidence that the active QS pheromone is processed to its mature form by a unique ABC transporter and not processed by an extracellular protease; and 3) supporting evidence of interspecies interactions between streptococci pheromones. Our results revealed the minimal pheromone scaffold needed for gallocin activation and uncovered unique interactions between two streptococci species QS signals that warrant further studies.