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Abstract：
Nodulation and nitrogen fixation genes of Mesorhizobium loti are encoded on the chromosome of the bacterium. Nevertheless, there is strong evidence that these genes can be transferred from an inoculant strain to nonsymbiotic mesorhizobia in the field environment. Here we report that the chromosomal symbiotic element of M. loti strain ICMP3153 is transmissible in laboratory matings to at least three genomic species of nonsymbiotic mesorhizobia. The element is 500 kb in size, integrates into a phe-tRNA gene, and encodes an integrase of the phage P4 family just within its left end. The entire phe-tRNA gene is reconstructed at the left end of the element upon integration, whereas the 3\' 17 nucleotides of the tRNA gene are present as a direct repeat at the right end. We termed the element a symbiosis island on the basis of its many similarities to pathogenicity islands. It may represent a class of genetic element that contributes to microbial evolution by acquisition.
摘要：
根瘤菌的结瘤和固氮基因在细菌的染色体上编码。然而，有强有力的证据表明，这些基因可以在田间环境中从接种菌株转移到非共生间根瘤菌。在这里，我们报告了M.loti菌株ICMP3153的染色体共生元素可在实验室交配中传播到至少三种非共生中根瘤菌的基因组物种。该元素的大小为500kb，整合到一个phe-tRNA基因中，并在其左端编码噬菌体P4家族的整合酶。整合后，整个phe-tRNA基因在元件的左端重建，而tRNA基因的3'17个核苷酸作为右端的直接重复存在。基于该元素与致病性岛的许多相似性，我们将其称为共生岛。它可能代表一类遗传元件，有助于通过采集进行微生物进化。
