The Enzyme Glucose-1-Phosphate Thymidylyltransferase RmlA Plays a Crucial Role in the Pathogenesis of Pectobacterium actinidiae GX1.

Pectobacterium actinidiae is one of the primary pathogens that causes summer canker disease in kiwifruit, yet its pathogenic mechanisms remain unknown. The exopolysaccharide PCAP-1a, isolated from the fermentation broth of P. actinidiae strain GX1, exhibits notable cytotoxicity and acts as a virulence factor facilitating host infection. Genome-wide analysis revealed a 21-gene cluster responsible for the biosynthesis of exopolysaccharides in GX1. Homologous recombination was used to systematically knock out these genes, which led to the identification of RmlA as a key protein in the synthesis of the PCAP-1a precursor. The deletion of the rmlA gene significantly affected the yield of PCAP-1a and resulted in a direct reduction in GX1 pathogenicity. Further studies revealed that mutations in the substrate binding site of RmlA weakened its capacity to bind G-1-P and dTTP, which led to markedly reduced pathogenicity in the corresponding complemented strains. This study indicates that the exopolysaccharide PCAP-1a serves as a virulence factor in the pathogenesis of GX1, and its biosynthesis depends on the polysaccharide synthesis gene rmlA and the substrate binding activity of its encoded protein.