United States Department of Agriculture, Agricultural Research Service, Molecular Plant Pathology Laboratory, Beltsville, Maryland, United States of America.
PLoS One. 2012;7(6):e38725. doi: 10.1371/journal.pone.0038725. Epub 2012 Jun 25.
Sinorhizobium meliloti strain 1021, a nitrogen-fixing, root-nodulating bacterial microsymbiont of alfalfa, has a 3.5 Mbp circular chromosome and two megaplasmids including 1.3 Mbp pSymA carrying nonessential 'accessory' genes for nitrogen fixation (nif), nodulation and host specificity (nod). A related bacterium, psyllid-vectored 'Ca. Liberibacter asiaticus,' is an obligate phytopathogen with a reduced genome that was previously analyzed for genes orthologous to genes on the S. meliloti circular chromosome. In general, proteins encoded by pSymA genes are more similar in sequence alignment to those encoded by S. meliloti chromosomal orthologs than to orthologous proteins encoded by genes carried on the 'Ca. Liberibacter asiaticus' genome. Only two 'Ca. Liberibacter asiaticus' proteins were identified as having orthologous proteins encoded on pSymA but not also encoded on the chromosome of S. meliloti. These two orthologous gene pairs encode a Na(+)/K+ antiporter (shared with intracellular pathogens of the family Bartonellacea) and a Co++, Zn++ and Cd++ cation efflux protein that is shared with the phytopathogen Agrobacterium. Another shared protein, a redox-regulated K+ efflux pump may regulate cytoplasmic pH and homeostasis. The pSymA and 'Ca. Liberibacter asiaticus' orthologs of the latter protein are more highly similar in amino acid alignment compared with the alignment of the pSymA-encoded protein with its S. meliloti chromosomal homolog. About 182 pSymA encoded proteins have sequence similarity (≤ E-10) with 'Ca. Liberibacter asiaticus' proteins, often present as multiple orthologs of single 'Ca. Liberibacter asiaticus' proteins. These proteins are involved with amino acid uptake, cell surface structure, chaperonins, electron transport, export of bioactive molecules, cellular homeostasis, regulation of gene expression, signal transduction and synthesis of amino acids and metabolic cofactors. The presence of multiple orthologs defies mutational analysis and is consistent with the hypothesis that these proteins may be of particular importance in host/microbe interaction and their duplication likely facilitates their ongoing evolution.
苜蓿中华根瘤菌 1021 株是一种固氮、结瘤的根瘤细菌微共生体，其拥有 3.5 Mbp 的环状染色体和两个大型质粒，包括携带非必需“辅助”固氮 (nif)、结瘤和宿主特异性 (nod) 基因的 1.3 Mbp pSymA。一种相关的细菌，经叶蝉传播的“亚洲韧皮杆菌”，是一种专性植物病原体，其基因组已被先前分析过，以确定与苜蓿中华根瘤菌环状染色体上同源基因的同源基因。一般来说，pSymA 基因编码的蛋白质在序列比对中与苜蓿中华根瘤菌染色体同源物编码的蛋白质更相似，而与“亚洲韧皮杆菌”基因组上携带的基因编码的同源蛋白更不相似。只有两种“亚洲韧皮杆菌”蛋白被鉴定为具有 pSymA 上编码的同源蛋白，但在苜蓿中华根瘤菌的染色体上没有编码。这两对同源基因对编码一种 Na(+)/K+反向转运蛋白（与家族 Bartonellacea 的细胞内病原体共享）和一种 Co++、Zn++和 Cd++阳离子外排蛋白，该蛋白与植物病原体根癌农杆菌共享。另一种共享蛋白，一种氧化还原调节的 K+外排泵可能调节细胞质 pH 和动态平衡。与 pSymA 编码蛋白与苜蓿中华根瘤菌染色体同源物的比对相比，后一种蛋白的 pSymA 和“亚洲韧皮杆菌”同源物在氨基酸比对中更相似。大约 182 个 pSymA 编码蛋白与“亚洲韧皮杆菌”蛋白具有序列相似性（≤ E-10），通常作为单个“亚洲韧皮杆菌”蛋白的多个同源物存在。这些蛋白参与氨基酸摄取、细胞表面结构、伴侣蛋白、电子传递、生物活性分子的输出、细胞内稳态、基因表达调控、信号转导以及氨基酸和代谢辅因子的合成。多个同源物的存在否定了突变分析，与这些蛋白可能在宿主/微生物相互作用中特别重要的假设一致，它们的复制可能促进了它们的持续进化。
