Studies Of Keto-acid Reductoisomerase (IlvC) In Xanthomonas Campestrispv. Campestris

Xanthomonas campestrispv.campestris belonging to the Xanthomonas,is a class of cruciferous plant pathogens.Xanthomonas campestrispv.campestris(Xcc),the causal agent of crucifer black rot,is possibly the most important disease of crucifers worldwide.As one of important phytopathogen,Xcc produces a range of virulence determinants.The production of these factors is regulated by quorum sensing(QS)mechanisms and mediated by molecules of the diffusible signal factor(DSF)family.Although the synthetic mechanism of DSF in Xanthomonas is relatively clear,the precursor of the branched signal molecules is not clear for the family of DSF signaling molecules,and the mechanism of the branched-chain fatty acids synthesis in Xanthomonas also remains to be demonstrated.Escherichia coli keto-acid reductoisomerase(Ec IlvC)is one of the key enzyme for the synthesis of branched-chain amino acids.Ketol-acid reductoisomerase(EC 1.1.1.86)(IlvC)catalyzes the reductive isomerisation of acetolactate to 2,3-dihydroxy-3-isovalerate or the conversion of 2-aceto-2-hydroxybutyrate to 2,3-dihydroxy-3-methylvalerate with N ADPH as cofactor,and introducing a branched amino acid synthesis pathway.The homologue ilv Cgene(Xcc ilv C)was also found in Xanthomonas.This study proved that Xcc IlvC in vitro was able to catalyze the formation of 2,3-dihydroxy-3-isovalerate,which was further utilized for branched fatty acid synthesis.Moreover,Xcc ilv C gene deletion mutant was auxotroph for branched-chain amino acids,and could not be synthesize branched-chain fatty acids.neither could syntheize the branched QS siganls DSF and IDSF.This study also showed that the Xcc ilv C gene mutation can significantly reduce the pathogenicity of Xanthomonas to the host plants.we found many metabolites were able to enhance the production of DSF family signals in Xcc resting cell.The above results illustrated the relationship between the synt hesis of branched chain fatty acid and the quorum sensing signal molecule.The function of Xcc IlvC in the synthesis of branched fatty acid,the synthesis of the quorum sensing signal molecular and the pathogenicity to the host plants was also studied.