中文摘要：

根据GenBank中致病疫霉菌无毒基因AVR3a序列（AJ893357）及参考文献中的特异引物（Pex147F，Pex147R），以致病疫霉菌基因组DNA为模板进行PCR扩增，得到了特异性扩增片段，并对特异片段进行纯化和测序。结果表明，该片段的长度为451bp，经序列分析表明，其氨基酸编码序列为441bp，共编码147个氨基酸。BLAST分析结果显示，该片段核苷酸序列与无毒基因AVR3a核苷酸序列的同源性为99．34％，氨基酸序列比对结果表明，氨基酸序列在第19、80和103位存在着差异，这些核苷酸序列差异以及某些关键氨基酸的改变有可能是导致不同菌株中AVR3a毒性变异的原因。

英文摘要：

Based on the AVR3a DNA sequence for Phytophthora infestans found in GenBank （accession No. AJ893357） and its primers （Pex147F, Pex147R） reported in references, PCR was carried out with the genomics DNA of P. infestans as template. The specific fragment was purified and cloned. The results showed that the fragment was 451bp. And, the analysis of sequence indicated that the coding region was 441bp and encoded 147 amino acids. The specific fragment sequence of P. infestans shared homology of 99.34% with AVR3a gene sequence. The alignments of amino acids showed that there were three amino acids at position 19, 80 and 103 that were different from that of AVR3a. This seemed to be the reason for the variation in virulence of AVR3a in different isolates of P. infestans.