Total Synthesis and Structural Revision of Rhabdobranin Reveals a Cryptic Gram-Negative Antibiotic.

Gram-negative bacteria present a major clinical challenge but also remain an underexplored source of antibacterial natural products. Resistance-guided genome mining of the entomopathogenic symbiont Xenorhabdus identified the rdb biosynthetic gene cluster, which encodes a putative prodrug antibiotic, pre-rhabdobranin. However, the inability to isolate the proposed active metabolite, rhabdobranin, has prevented direct functional evaluation. Here we report a convergent total synthesis of the proposed structure of pre-rhabdobranin B, which revealed a stereochemical misassignment at the N-terminal arginine residue. Synthesis of both rhabdobranin epimers showed that, although they are nearly indistinguishable by standard analytical methods, inversion at this single stereocenter has a pronounced effect on antibacterial activity. Biological evaluation of the revised rhabdobranin structure revealed potent antibacterial activity against Gram-negative pathogens, including WHO critical-priority carbapenem-resistant Klebsiella pneumoniae. Cellular and biochemical profiling implicated inhibition of protein biosynthesis as its principal antibacterial mechanism. We further show that the GNAT-family acetyltransferase RdbK N-acetylates rhabdobranin, attenuating its activity and establishing a secondary self-resistance mechanism. These findings validate resistance-gene-guided discovery in Gram-negative symbionts as a strategy for uncovering cryptic antibiotics and identify rhabdobranin as a promising scaffold for Gram-negative antibiotic development.