LipB-LipA is the only lipoyl acylation pathway in Xanthomonas campestris

Abstract:

[Objective] Cofactor lipoic acid participates in many basic cellular metabolic processes. Xanthomonas campestris pv. campestris (Xcc) is the pathogen of black rot of cruciferous plants, causing plant disease all over the world. Studies on the synthetic mechanism of lipoic acid in Xcc will facilitate the discovery of novel methods to control the disease.[Methods] XC_0713 (XccLipA) and XC_0712 (XccLipB) in Xcc were found based on the sequence alignments with E. coli LipA and LipB, the key enzymes for lipoic acid synthesis. For complementation analysis, XcclipA and XcclipB were amplified and ligated into expression vector pBAD24M, which were further transferred into E. coli mutants, respectively. The growth of transformants was analyzed. With the aid of E. coli lplA expression, XcclipA and XcclipB were deleted in the genome by homologous recombination, respectively. The growth and virulence of the two mutants were analyzed.[Results] XcclipA and XcclipB complemented the growth of E.coli fabA and fabB mutants on the minimal media, respectively. Both XcclipA and XcclipB are essential genes, which cannot be deleted directly. However, XcclipA and XcclipB could be deleted when pSRK-EclplA was introduced. The two mutants could not grow on the minimal medium, but grew well when lipoic acid supplemented. XcclipB mutant could grow on the nutritional medium. XcclipA mutant almost abolished the virulence to the host plant, but XcclipB mutant showed similar virulence with wild-type strain.[Conclusion] XcclipA codes lipoyl synthase, and XcclipB codes octanoyl transferrase, both are essential genes for Xcc growth. The LipB-LipA is the only lipoic acylation pathway in Xcc. XcclipA deletion mutant showed no virulence to host plant, indicating XccLipA is a promising target for anti-bacteria agent selection.