梨火疫病病原菌（Erwinia amylovora）三型分泌系统的鉴别及Erwinia spp. HrpA的分析

 【目的】明确梨火疫病病原菌（Erwinia amylovora ）三型分泌系统（type Ⅲ secretion system，TTSS）的所在区域、相关基因及Erwinia spp.的HrpA（hypersensitive response and pathogenicity gene A）选择压水平。【方法】利用生物信息学方法，通过BLAST程序对梨火疫病菌基因组的核苷酸数据库进行同源比对，同时对Erwinia spp.与宿主互作的表面蛋白HrpA进行选择压分析。【结果】TTSS分析结果显示在40 kb大小的毒力岛上有27个TTSS相关基因。通过与看家基因及毒力岛上的致病性相关的重要基因HrpN的比较、选择压分析、多序列比较，发现HrpA基因处于较强的选择压作用下，且HrpA蛋白N端主要受正向选择，C端主要受净化选择。NJ法构建的HrpA系统发育树显示Erwinia spp.形成两个明显的分支，表明HrpA基因可能在种间分化后产生了不同的选择压变化。【结论】梨火疫病病原菌基因组3.14－3.18 Mb为其TTSS分布区域。该病原菌通过TTSS侵染寄主植物，在其病原菌表面有一种类鞭毛结构的TTSS蛋白HrpA，HrpA作为传输者起着将效应分子输送到宿主内部的功能，在进化上受到了较强的选择压影响。

Identification of Type Ⅲ Secration System in Erwinia amylovora and Analyse of HrpA in Erwinia spp.

【Objective】 The objective of this study was to investigate the TTSS in Erwinia amylovora and the HrpA selection pressure in Erwinia spp. 【Method】 Bioinformatics strategies were used in this study by BLAST scan to search the Erwinia amylovora genomic nucleotide database and compare their homology, and analyze the selection pressure about the surface protein HrpA which is associated with the interaction between Erwinia spp. and the hosts. 【Result】 Twenty-seven TTSS- like proteins were identified in the 40 kb pathogenecity island. In contrast, the selection pressure value for HrpA was strongly elevated by comparison with those of housekeeping genes and HrpN. The comparison among multi HrpA sequences showed that most of the HrpA N terminus were subjected to positive selection and the C terminus were subjected to purifying selection. Erwinia spp. phylogenetic tree based on HrpA showed that there were two distinct groups. Analysis indicated that different modes of selection operated after separation of species. 【Conclusion】 The TTSS in Erwinia amylovora lies between 3.14 Mb to 3.18 Mb and the pathogen uses this system to invade the host. At the bacterial cell surface, the TTSS has a filamentous-like structure, HrpA, which serves as a conduit for the transfer of effectors into the plant cell. HrpA has a strong selection pressure in its evolutionary history. This paper has some positive significance in revealing the host range and plant resistance.