ABSTRACT
Xanthomonas campestris pv. campestris (Xcc), the pathogenic agent of black rot disease in cruciferous plants, produces large amount of extracellular polysaccharide (EPS), which has found wide applications in industry. For the great commercial value of the xanthan gum, many of the genes involved in EPS biosynthesis have been cloned and the mechanism of EPS biosynthesis also has been studied. In order to clone genes involved in EPS biosynthesis, Xcc wild-type strain 8004 was mutagenized with transposon Tn5 gusA5, and a number of EPS-defective mutants were isolated in our previous work. The Tn5 gusA5 inserted sites of these mutants were located by using thermal asymmetric interlaced PCR, and results showed that two EPS-defective mutants were insertion mutants of the gene wxcA which involved in lipopolysaccharide (LPS) biosynthesis. The gene wxcA involved in lipopolysaccharide biosynthesis but dose not extracellular polysaccharide in others' report. wxcA::Tn5 gusA5 mutant 021C12, the polar mutant, was complemented with recombinant plasmid pLATC8570 harboring an intact wxcA gene in this work, but the yield of EPS of the wxcA::Tn5 gusA5 mutant was not restored. In order to identify the function of wxcA gene of Xcc 8004 strain, the gene wxcA was deleted by gene replacement strategy, and the no-polar mutant of wxcA was obtained. DeltawxcA mutant strain, named Xcc 8570, was confirmed by using both PCR and southern analysis. Beside the LPS biosynthesis of deltawxcA mutant was affected, The EPS yield of deltawxcA mutant strain reduced by 50% as compared with the wild-type strain 8004. DeltawxcA mutant could be complemented in trans with the intact wxcA gene, and the EPS yield of the mutant was restored. The combined data showed that wxcA gene not only involved in LPS biosynthesis but also EPS yield in Xcc 8004 strain.