Spermidine targets GGA-rich small RNAs for virulence regulation.

Spermidine (SPD), a polyamine essential for various biological processes, has long been recognized as a signaling molecule. Yet, its specific molecular targets and mechanisms of action remain elusive. In this study, we uncover a previously unknown mechanism whereby SPD selectively binds to small regulatory RNAs, driving their degradation and subsequently modulating virulence in Pseudomonas aeruginosa. We identified GGA-rich SPD-binding RNA motifs in RsmZ and RsmY, two small RNA regulators of the type III secretion system (T3SS) and quorum sensing pathways and demonstrated a motif-dependent interaction with SPD. SPD binding alters the structural properties of these RNAs in a manner associated with reduced RNA stability, promoting their degradation, at least in part, through the CafA endonuclease. In vivo, this process enhances T3SS activity and suppresses quorum sensing, thereby facilitating the acute infection of P. aeruginosa. These findings highlight a previously unrecognized regulatory role of polyamine-small RNA interactions in bacterial virulence, offering potential targets for therapeutic interventions.