十字花科黑腐病是由野油菜黄单胞菌野油菜黑腐病致病变种引起的，严重危害十字花科蔬菜的生产。脂肪酸合成是细菌的基础代谢之一，与细菌的生命活动密切相关。野油菜黄单胞菌含有大量的支链脂肪酸，另外作为该菌群体感应调控的信号分子DSF本身也是一种特殊的支链脂肪酸。然而到目前为止有关该菌支链脂肪酸的合成机制以及与DSF合成的关系仍不清楚。研究表明3-酮脂酰ACP合成酶III（FabH）的底物专一性决定着细菌能否合成支链脂肪酸。本项目围绕野油菜黄单胞菌的FabH，开展以下三方面的研究：（1）FabH在野油菜黄单胞菌脂肪酸合成中的功能及催化机制；（2） FabH与DSF信号合成的关系；（3）FabH在中链脂酰CoA再利用中的作用。研究目标是揭示野油菜黄单胞菌FabH的生物学功能，预期的成果有助于寻找防治黑腐病新的靶点，为进一步研发高效、特异的新一代绿色农药奠定基础。

Xanthomonas campestris pv. campestris is the causal agent of black rot of crucifers, which is a considerable challenge for the production of cruciferous vegetables. Fatty acid synthesis (FAS) is a primary bacterial metabolic pathway and hence is essential for survival of bacteria. Xcc has a complex fatty acid composition comprised of straight-chain saturated fatty acids, unsaturated fatty acids, and branched-chain fatty acids (BCFA). BCFAs account for about 50% of the total fatty acid. Xcc also produces diffusible signal factor (DSF), which is a 2-enoic acids, as the quorum-sensing (QS) signal to communicate with each other to facilitate their adaptation to changing environmental conditions. However, up to date it is little known about the mechanism of BCFA synthesis and relationship between DSF biosynthesis and fatty acid synthesis pathway in Xcc. It has been demonstrated that bacteria FabH (3-ketoacyl-acyl carrier protein synthase III) catalyzes the key initiation step of bacteria fatty acid synthesis, which substrate specificity is a major determining factor for branched-chain fatty acids synthesis. In this proposal, we focus on Xcc FabH and set out to investigate three important and interesting questions: (1) to characterize the catalytic mechanism of FabH in fatty acid synthesis of Xcc; (2) to identify the function of FabH in DSF synthesis; (3) to investigate the mechanism of FabH in recycle of medium-chain acyl-CoAs. The goal of this project is to understand the biological function of Xcc FabH. It is expected that these research will provide some references for discovering new target to inhibit Xcc growth and developing novel biopesticides to effectively control black rot of crucifers.