[Identification and cloning of a novel gene involved in EPS biosynthesis of Xanthomonas campestris pv. campestris].

Xanthomonas campestris pv. campestris ( Xcc), causative agent of the black rot disease of cruciferous crops worldwide, produces large amount of extracellular polysaccharide( EPS), which has found wide applications in industry. In order to clone genes involved in EPS biosynthesis, Xcc wild-type strain 8004 was mutagenized with transposon Tn5gus A5, and a number of EPS-defective mutants were isolated. The Tn5gusA5 insertion sites in the mutants were analyzed by using thermal asymmetric interlaced PCR(TAIL-PCR), and the corresponding genes were identified by homology blast to the completely sequenced genome of Xcc 8004 strain. A novel gene, waxE, identified from the EPS-defective mutant 151D09, was found to be disrupted by the insertion of Tn5gusA5 in the open reading frame(ORF) with genome coordinates 4478998bp to 4479819bp.This gene showed 52% similarity to the kdtX gene of Serratia marcescens and 50% to the waaE of Klebsiella pneumoniae at amino acid level, with characteristics of glycostransferase 2 family domain. In order to identify the function of waxE gene, waxE gene deletion mutant of Xcc 8004 was constructed by gene replacement strategy in which waxE gene of genome was replaced by kanamycin resistant gene kan. The waxE gene deletion mutant strain, named Xcc 8570, was confirmed by both PCR and southern analysis. The growth rate of the deletion mutant 8570 in rich medium was not affected, but the EPS yield reduced by 35% as compared with the wildtype strain 8004. The deletion mutant could be completmented in trans with plasmid pLATC8976 harboring an intact waxE gene, and the EPS yield of the mutant was restored. The combined data showed that waxE gene involved in EPS biosynthesis in Xcc.