The FabA-FabB Pathway is not Essential for Unsaturated Fatty Acid Synthesis, but Modulates DSF Synthesis in Xanthomonas campestris pv. campestris

Most bacteria use type II fatty acid synthesis (FAS II) systems for synthesizing fatty acids, of which the conserved FabA-FabB pathway is considered to be crucial for unsaturated fatty acid (UFA) synthesis in gram-negative bacteria. Xanthomonas campestris pv. campestris (Xcc), the phytopathogen of black-rot disease in crucifers, produces higher quantities of UFAs under low temperature conditions for increasing membrane fluidity. The fabA and fabB genes were identified in the Xcc genome by BLAST analysis; however, the growth of the Xcc fabA and fabB deletion mutants was comparable to that of the wild-type strain in nutrient and minimal media. The Xcc ΔfabA and Xcc ΔfabB strains produced large quantities of UFAs, and altogether these results indicated that the FabA-FabB pathway is not essential for growth or UFA synthesis in Xcc. We also observed that the expression of Xcc fabA and Xcc fabB restored the growth of the temperature-sensitive Escherichia coli fabA and fabB mutants, CL104 and CY242, respectively, under non-permissive conditions. The in vitro assays demonstrated that the FabA and FabB proteins of Xcc catalyzed fatty acid synthesis. Our study also demonstrated that the production of diffusible signal factor (DSF) family signals that mediate quorum sensing (QS) was higher in the Xcc ΔfabA and Xcc ΔfabB strains and greatly reduced in the complementary strains, which exhibited reduced swimming motility and attenuated host plant pathogenicity.