Serine Acetyltransferase from Pseudomonas aeruginosa: Distinctive Features, Pleiotropic Roles, and Therapeutic Potential.

Cysteine biosynthesis is increasingly recognized as a critical determinant of bacterial virulence, highlighting this pathway as a promising reservoir of novel antimicrobial targets. In Pseudomonas aeruginosa, however, the molecular basis of cysteine production has only recently begun to emerge. Here, we identify PA3816 as the major P. aeruginosa serine acetyltransferase (PaCysE), the enzyme responsible for generating the activated serine intermediate that feeds O-acetylserine sulfhydrylase-mediated cysteine synthesis. Through a combination of biochemical and genetic approaches, we demonstrate that PaCysE efficiently catalyzes L-serine acetylation in vitro, and in turn, deletion mutants exhibit cysteine auxotrophy, underscoring its essential contribution to O-acetylserine production. Notably, PaCysE is less sensitive to feedback inhibition by cysteine and does not appear to form the canonical cysteine synthase complex, suggesting a regulatory architecture that diverges from well-characterized orthologs. Loss of PaCysE function has broad physiological consequences, including enhanced biofilm formation, reduced pyocyanin production, and attenuated infectivity in an animal model, linking cysteine biosynthesis directly to pathogen fitness. Finally, we identify a thiazole derivative that inhibits PaCysE activity (IC50 ≈ 30 µM) and suppresses bacterial growth in a cysteine-dependent manner, providing a proof-of-concept for therapeutically targeting this pathway.