Fuctional Analysis Of Hisa And Hisc Related To Pathogenicity In Acidovorax Citrulli

Acidovorax citrulli is an important bacteria which infects watermelon, melon and other cucurbits. It mainly damages watermelon and melon, causes leaf blight and fruit rot and even total losses. But, most studies on Ac were related to its identification, biological characteristics and disease occurrence rule, few report focused on its pathogenicity and related genes. So this study primarily explored the pathogenicity-related genes of Acidovorax citrulli.In our lab, we generated a transposon (mini-Tn5) mutant library on the background of strain xj112 of Ac. We also got a mutant which could not induce the hypersensitive response(HR) on nonhost tobacco while its virulence reduced significantly. It was identified as hisC gene through subclone techonology which encoded a protein histidinol-phosphate aminotransferase. As a kind of histidine synthase, it catalytics the seventh step of the reaction. We also constructed the mutation of hisC gene through double-exchange in homologous recombination and analysised its function.Histidine is the essential amino acids in protein synthesis of the organisms. Its biosynthetic pathway has got widely identification. There are nine genes in the his-genes of Ac. And according to the order in histidine synthesis, they respectively are hisG,hisE,hisI,hisA,hisF,hisH,hisB,hisC and hisD. However, there is few report on his-genes of Ac at present. Changes of pathogenicity may be caused by a variety of reasons. Histidine as an essential amino acid, if an enzyme in histidine synthesis process is destroyed, then whether the pathogenicity of Ac will be affected or not. Based on the close relationship between the pathogenicity of Ac and hisC gene, and in order to verify the relationship between the synthesis of histidine and the pathogenicity of Ac, we studied the role of another enzyme in histidine synthesis process. It is 1-(5-phosphoribosyl)-5-[(5-phosphoribosylamino)methyli- deneamino]imidazole-4-carboxamide isomerase and encoded by the hisA gene. We constructed the insertion mutant and studied the characteristics of hisA gene through single cross.The results show that these mutants could not elicit the hypersensitive response(HR) in nonhost tobacco, their virulence reduce significantly and disease symptome caused by mutants was delayed about 48 hours when compared to the wild type strain. By adding exogenous histidine, the characteristics of the mutants could recover. It confirmed that the changes of the characteristics of the mutants are directly related to the synthesis of histidine. And we initially studied the function of his genes in Ac.