A novel multifunctional glycosidase inhibitor from plant pathogenic bacterium Pseudomonas syringae: its discovery, biosynthesis and biological roles
N. SANGUANKIATTICHAI, B. CHANDRASEKAR, N. HARDENBROOK, Y. SHENG, D. KRAHN, W. W. A. TABAK, P. BUSCAILL, M. KAISER, P. ZHANG, G. M. PRESTON, R. A. L. VAN DER HOORN
Department of Biology, University of Oxford, Oxford, OX1 3RB, United Kingdom

During infection, plant-secreted hydrolases play a prominent role in defence and are often targeted by pathogens. We recently discovered that Pseudomonas syringae produces a small molecule inhibitor ‘galactosyrin’ to suppress the plant-secreted Beta-galactosidase BGAL1. Here, we identified the inhibitor structure and elucidated its biosynthesis and biological roles. Through a genetic screen and comparative genomics, we identified the biosynthetic gene cluster and its regulators, and confirmed that this cluster confers inhibitor biosynthesis when transformed into E. coli. Biochemical characterisation and modeling of encoded enzymes identified a novel biosynthetic pathway that we could reconstruct in vitro. The inhibitor structure was identified by cryo-electron microscopy of the inhibitor-enzyme complex at 1.9 angstrom resolution, revealing a novel iminosugar structure in the active site, which was further confirmed by mass spectrometry of the isolated inhibitor. A synthetic version of this inhibitor has identical chemical characteristics and inhibits BGAL1 and other glycosidases. Further profiling of the proteome and metabolome of infected plants showed that the inhibitor promotes the accumulation of certain glycoproteins and glycosides in the apoplast during infection. Our work has elucidated a novel natural product, biosynthetic pathway and biological activities that highlight plant glycobiology manipulation by pathogens and provide leads for future biotechnological applications.