A novel 3‐oxoacyl‐ACP reductase (FabG3) is involved in the xanthomonadin biosynthesis of Xanthomonas campestris pv. campestris

Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot in crucifers, produces a membrane‐bound yellow pigment called xanthomonadin to protect against photobiological and peroxidative damage, and uses a quorum‐sensing mechanism mediated by the diffusible signal factor (DSF) family signals to regulate virulence factors production. The Xcc gene XCC4003, annotated as Xcc fabG3, is located in the pig cluster, which may be responsible for xanthomonadin synthesis. We report that fabG3 expression restored the growth of the Escherichia coli fabG temperature‐sensitive mutant CL104 under non‐permissive conditions. In vitro assays demonstrated that FabG3 catalyses the reduction of 3‐oxoacyl‐acyl carrier protein (ACP) intermediates in fatty acid synthetic reactions, although FabG3 had a lower activity than FabG1. Moreover, the fabG3 deletion did not affect growth or fatty acid composition. These results indicate that Xcc fabG3 encodes a 3‐oxoacyl‐ACP reductase, but is not essential for growth or fatty acid synthesis. However, the Xcc fabG3 knock‐out mutant abolished xanthomonadin production, which could be only restored by wild‐type fabG3, but not by other 3‐oxoacyl‐ACP reductase‐encoding genes, indicating that Xcc FabG3 is specifically involved in xanthomonadin biosynthesis. Additionally, our study also shows that the Xcc fabG3‐disrupted mutant affects Xcc virulence in host plants.