宿主诱导共生岛属间水平转移分子机理的研究

Horizontal gene transfer (HGT) allows bacteria to rapidly acquire complex new traits, and it has been and remains a key driving forse in bacterial evolution. Since the first genomic islands (GEIs) was found in E.coli, it has become evident that conjugation systems are widespread in chromosome-borne mobile genetic elements. Though pathogenicity islands (PAIs) was well studied in all kinds of GEIs, we have less known about symbiosis islands in Rhizobia. Our work found that symbiosis islands in Azorhizobium caulinodans ORS571 can transfer to other rhizobia, and the recipient rhizobia obtian the new trait to form nodules with Sesbania rostrata. We also found that the transfer ratio was raised nearly 1000 times when host exists. Based on these work, two specific aims are outlined in this proposal to investigate the mechanism of symbiosis islands transfer. First, we will study why and how host promotes the transfer ratio of symbiosis islands. Second, we have developed a high through genetic seletion method allowing us to monitor genes in recipient cells which can improve or inhibit cells receive symbiosis islands. Using this novel approach, we will shed light on currently little known about genctic charateristic of recipient cells in GEIs. By understanding the complete genetic properties of symbiosis islands transfer, we can illuminate the evolution track of rhizobia. The ultimate goal of this research is to gain knowledge that will lead to broaden the host specificity of rhizobia.

基因水平转移能够提高原核生物对环境的适应性，丰富细菌遗传多样性，自1990年发现细菌基因岛之后，关于基因岛水平转移的研究引起广泛地关注，主要集中在病原细菌毒力岛基因水平转移上，而根瘤菌共生相关基因水平转移的研究仅有少量报道。申请人前期的研究发现田菁根瘤菌A.caulinodans ORS571共生岛能转移到不同属的根瘤菌中，并能使受体菌株获得在田菁上结瘤和固氮的能力，扩大了根瘤菌的宿主范围，并且在宿主存在时水平转移的频率提高1000倍。因此申请人将在此基础上开展以下研究：1.宿主植物的信号分子如何提高共生岛基因水平转移的频率及其机制；2.受体根瘤菌中影响共生岛基因水平转移相关基因的筛选，阐明限制根瘤菌接受外源共生岛基因的因素及分子机制。本项目的开展可以对扩大根瘤菌的宿主范围，阐明根瘤菌的进化机理提供理论依据。

项目针对田菁根瘤菌共生岛跨属水平转移的现象开展分子机理研究，主要从宿主植物促进田菁根瘤菌共生岛基因水平转移机理的研究和不同受体菌株中影响共生岛水平转移相关基因的筛选和分子机理的研究开展研究工作。项目揭示了宿主通过柚皮素与共生岛上AhaR蛋白结合，激活整合酶表达，从而促进共生岛的水平转移。同时发现供体菌中的转移接合相关的tra、trb以及整合酶基因int能影响受体根瘤菌接受共生岛，并筛选到不能接受共生岛的菜豆根瘤菌CFN42中一个XRE家族调控蛋白，该蛋白缺失后菜豆根瘤菌能接受共生岛，并在田菁上结瘤。该项目揭示了宿主通过柚皮素一方面诱导田菁根瘤菌结瘤相关基因的表达，同时诱导共生岛上AhaR蛋白的活性，促进共生岛向其它根瘤菌转移，扩大宿主根际能结瘤的根瘤菌种群，从而保证共生固氮的顺利进行。