The role of translational product from avirulent genes in xanthomonads bacteria after traficking into plant nuclei.

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It has been known that avr/pthA gene family of Xanthomonads bacteria induce either resistant reactions or disease development. Since they are highly homologous each other, the basic mechanisms involved in both reactions of plants may be common. This study is to find out the initial event of both reactions using molecular biological analyses. Using Xanthomonas citri, a causal agent of citrus canker, as the model system, we have cloned three pthA homologues, and found that only one of them has the ability of causing canker symptom. Comparison of the sequences of these homologues showed that they showed complete homology except in the tandemly repeating region within their genes. Thus, this region was shown to be responsible for the formation of canker symptom on citrus plant, and the importance of the secondary structure was also shown to be important. After overexpression of pthA in E. coli using expression vector, the purified protein was used for affinity chromatography to isolate its receptor protein from citrus leaves. N-terminal amino acid sequence of this receptor protein was determined. Using the mixed oligomer for this peptide was used for the isolation of the gene for the receptor protein. Its nucleotide sequence was determined and it was found to be highly homologous to CCoAMT (S-adenosyl L methionine : trans cafeoryl CoenzymeA 3-O-methyltransferase) of various plants. This enzyme has been shown to be responsible for the lignin after infection with pathogen. Thus, the mechanism for the canker formation on citrus plants was suggested to be due to the alteration in the lignin synthesis which leads to cell elongation.