Evolution of Saxitoxin Synthesis in Cyanobacteria and Dinoflagellates
Evolution of Saxitoxin Synthesis in Cyanobacteria and Dinoflagellates
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DOI：
10.1093/molbev/mss142
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发表时间：
2013-01-01
影响因子：
10.7
通讯作者：
Erdner, Deana L.
中科院分区：
文献类型：
--
作者：
Hackett, Jeremiah D.;Wisecaver, Jennifer H.;Erdner, Deana L.
甲藻产生多种有毒的次生代谢物，对海洋生态系统和渔业产生重大影响。石房蛤毒素 (STX) 是麻痹性贝类中毒的原因，由三种海洋甲藻属产生，也由一些淡水蓝藻产生。蓝细菌中涉及 STX 合成的基因已被鉴定，但在甲藻的大量基因组中尚未有报道。我们收集了几种产生 STX 的甲藻和相关无毒物种的全面转录组数据集，并鉴定了 13 个蓝藻 STX 合成基因的 265 个推定同源物，包括直接参与毒素合成的所有基因。四种蛋白质的假定同源物在蓝细菌的系统发育中密切相关，并且可能是甲藻中 sxtA、sxtG 和 sxtB 的功能同源物。然而，系统发育并不支持这些基因在有毒蓝细菌和甲藻之间直接转移。 SxtA 在甲藻中分裂成两个蛋白质，对应于包含甲基转移酶和酰基载体蛋白结构域的 N 端部分和包含转氨酶结构域的 C 端部分。 sxtB 和 N 端 sxtA 的同源物存在于无毒菌株中，表明它们的功能可能不仅限于石房蛤毒素的产生。仅在有毒菌株中发现了 sxtA 和 sxtG C 末端的同源物。需要对 STX+ 甲藻进行更彻底的调查，以确定这两个基因是否可能是甲藻中 SXT 生产所特有的。 A. tamarense 转录组不包含其余 STX 基因的同源物。尽管如此，我们还是确定了具有相似预测生化活性的候选基因，这些基因可以解释缺失的功能。这些结果表明，STX 合成途径可能是在关系较远的蓝细菌和甲藻中独立组装的，尽管使用了一些进化相关的蛋白质。 STX 在蓝细菌或甲藻中的生物学作用尚不清楚。然而，这两个生态上不同的生物体中的 STX 产生表明，这种毒素为生产者带来了我们尚未完全了解的好处。
Dinoflagellates produce a variety of toxic secondary metabolites that have a significant impact on marine ecosystems and fisheries. Saxitoxin (STX), the cause of paralytic shellfish poisoning, is produced by three marine dinoflagellate genera and is also made by some freshwater cyanobacteria. Genes involved in STX synthesis have been identified in cyanobacteria but are yet to be reported in the massive genomes of dinoflagellates. We have assembled comprehensive transcriptome data sets for several STX-producing dinoflagellates and a related non-toxic species and have identified 265 putative homologs of 13 cyanobacterial STX synthesis genes, including all of the genes directly involved in toxin synthesis. Putative homologs of four proteins group closely in phylogenies with cyanobacteria and are likely the functional homologs of sxtA, sxtG, and sxtB in dinoflagellates. However, the phylogenies do not support the transfer of these genes directly between toxic cyanobacteria and dinoflagellates. SxtA is split into two proteins in the dinoflagellates corresponding to the N-terminal portion containing the methyltransferase and acyl carrier protein domains and a C-terminal portion with the aminotransferase domain. Homologs of sxtB and N-terminal sxtA are present in non-toxic strains, suggesting their functions may not be limited to saxitoxin production. Only homologs of the C-terminus of sxtA and sxtG were found exclusively in toxic strains. A more thorough survey of STX+ dinoflagellates will be needed to determine if these two genes may be specific to SXT production in dinoflagellates. The A. tamarense transcriptome does not contain homologs for the remaining STX genes. Nevertheless, we identified candidate genes with similar predicted biochemical activities that account for the missing functions. These results suggest that the STX synthesis pathway was likely assembled independently in the distantly related cyanobacteria and dinoflagellates, although using some evolutionarily related proteins. The biological role of STX is not well understood in either cyanobacteria or dinoflagellates. However, STX production in these two ecologically distinct groups of organisms suggests that this toxin confers a benefit to producers that we do not yet fully understand.