Insights into the Evolution of Vitamin B12 Auxotrophy from Sequenced Algal Genomes
Insights into the Evolution of Vitamin B12 Auxotrophy from Sequenced Algal Genomes
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DOI：
10.1093/molbev/msr124
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发表时间：
2011-10-01
影响因子：
10.7
通讯作者：
Smith, Alison G.
中科院分区：
文献类型：
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作者：
Helliwell, Katherine E.;Wheeler, Glen L.;Smith, Alison G.
维生素B-12（钴胺素）是人类的膳食必需品，因为它是两种酶（甲基丙二酰辅酶A β和甲硫氨酸合酶（METH））的必需辅因子。陆生植物和真菌既不合成也不需要钴胺素，因为它们不含甲基丙二酰辅酶A β，并且具有替代的不依赖于B-12的甲硫氨酸合酶（METE）。在藻类王国中，大约一半的微藻物种需要维生素作为生长补充剂，但这些物种之间没有系统发育关系，这表明营养缺陷型在进化过程中多次出现。我们着手确定这一观察的基本细胞机制，通过调查元素的B-12代谢的15种不同的藻类的基因组测序，与代表的红色，绿色，褐藻，硅藻和颗石藻，包括大型和微型藻类，和海洋和淡水环境。从这个分析中，连同生长测定，我们发现了功能性METE基因的缺失和B-12营养缺陷型之间的强相关性。在依赖于B-12的绿色Volvox carteri和Gonium pectorale（与不依赖于B-12的莱茵衣藻有亲缘关系）中存在METE单一假基因，这表明这些谱系中最近进化出了B-12依赖性。在两个C.对于莱茵衣藻和三角褐指藻，在钴胺素存在下的生长导致METE转录的抑制，提供了基因丢失的机制。因此，不同的环境条件可能是这些生物体中B-12营养缺陷型的多个独立起源的原因。由于钴胺素的最终来源是来自原核生物，在不同的藻类谱系中METE的选择性损失将对这些生物体在依赖细菌方面产生重要的生理和生态后果。
Vitamin B-12 (cobalamin) is a dietary requirement for humans because it is an essential cofactor for two enzymes, methylmalonyl-CoA mutase and methionine synthase (METH). Land plants and fungi neither synthesize or require cobalamin because they do not contain methylmalonyl-CoA mutase, and have an alternative B-12-independent methionine synthase (METE). Within the algal kingdom, approximately half of all microalgal species need the vitamin as a growth supplement, but there is no phylogenetic relationship between these species, suggesting that the auxotrophy arose multiple times through evolution. We set out to determine the underlying cellular mechanisms for this observation by investigating elements of B-12 metabolism in the sequenced genomes of 15 different algal species, with representatives of the red, green, and brown algae, diatoms, and coccolithophores, including both macro- and microalgae, and from marine and freshwater environments. From this analysis, together with growth assays, we found a strong correlation between the absence of a functional METE gene and B-12 auxotrophy. The presence of a METE unitary pseudogene in the B-12-dependent green algae Volvox carteri and Gonium pectorale, relatives of the B-12-independent Chlamydomonas reinhardtii, suggest that B-12 dependence evolved recently in these lineages. In both C. reinhardtii and the diatom Phaeodactylum tricornutum, growth in the presence of cobalamin leads to repression of METE transcription, providing a mechanism for gene loss. Thus varying environmental conditions are likely to have been the reason for the multiple independent origins of B-12 auxotrophy in these organisms. Because the ultimate source of cobalamin is from prokaryotes, the selective loss of METE in different algal lineages will have had important physiological and ecological consequences for these organisms in terms of their dependence on bacteria.