固氮“藻青菌”UCYN-A基因组测序完成
被称为UCYN-A的迄今尚未培养出的固氮“藻青菌”广泛分布在世界海洋中。元基因组分析表明,它没有为光合机构中产生氧气的光合体系II复合物(该复合物帮助它在日光下固氮)编码的基因,也没有固碳基因。
现在,利用大规模并行配对端焦磷酸测序技术,研究人员确定了完整的UCYN-A基因组。原来,它是一种特别简单的生物,缺少很多核心代谢通道,严重依赖于其他生物获取有机碳、甚或有机含氮化合物。虽然该基因组与叶绿体和内共生体在结构上有相似性,但对自然种群所做的实验迄今未能发现与其他微生物的任何共生关系。
原始出处:
Nature 464, 90-94 (4 March 2010) | doi:10.1038/nature08786
Metabolic streamlining in an open-ocean nitrogen-fixing cyanobacterium
H. James Tripp1, Shellie R. Bench1, Kendra A. Turk1, Rachel A. Foster1, Brian A. Desany2, Faheem Niazi2, Jason P. Affourtit2 & Jonathan P. Zehr1
1 Ocean Sciences Department, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
2 454 Life Sciences, a Roche Company, 20 Commercial Street, Branford, Connecticut 06405, USA
Nitrogen (N2)-fixing marine cyanobacteria are an important source of fixed inorganic nitrogen that supports oceanic primary productivity and carbon dioxide removal from the atmosphere1. A globally distributed2, 3, periodically abundant4 N2-fixing5 marine cyanobacterium, UCYN-A, was recently found to lack the oxygen-producing photosystem?II complex6 of the photosynthetic apparatus, indicating a novel metabolism, but remains uncultivated. Here we show, from metabolic reconstructions inferred from the assembly of the complete UCYN-A genome using massively parallel pyrosequencing of paired-end reads, that UCYN-A has a photofermentative metabolism and is dependent on other organisms for essential compounds. We found that UCYN-A lacks a number of major metabolic pathways including the tricarboxylic acid cycle, but retains sufficient electron transport capacity to generate energy and reducing power from light. Unexpectedly, UCYN-A has a reduced genome (1.44?megabases) that is structurally similar to many chloroplasts and some bacteria, in that it contains inverted repeats of ribosomal RNA operons7. The lack of biosynthetic pathways for several amino acids and purines suggests that this organism depends on other organisms, either in close association or in symbiosis, for critical nutrients. However, size fractionation experiments using natural populations have so far not provided evidence of a symbiotic association with another microorganism. The UCYN-A cyanobacterium is a paradox in evolution and adaptation to the marine environment, and is an example of the tight metabolic coupling between microorganisms in oligotrophic oceanic microbial communities.
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