Institute of Biodiversity and Environmental Conservation, Universiti Malaysia Sarawak, Kota Samarahan 94300, Sarawak, Malaysia.
Bachok Marine Research Station, Institute of Ocean and Earth Sciences, University of Malaya, Bachok 16310, Kelantan, Malaysia.
Harmful Algae. 2016 Jun;56:9-21. doi: 10.1016/j.hal.2016.04.005. Epub 2016 May 17.
Saxitoxins (STXs) constitute a family of potent sodium channel blocking toxins, causative agents of paralytic shellfish poisoning (PSP), and are produced by several species of marine dinoflagellates and cyanobacteria. Two STX-core genes, sxtA and sxtG, have been well elucidated in Alexandrium but the expression of these genes under various nutritional modes in tropical species remains unclear. This study investigates the physiological responses of a tropical Pacific strain of Alexandrium minutum growing with nitrate or ammonium, and with various nitrogen to phosphorus (N:P) supply ratios. The transcriptional responses of the sxt genes were observed. Likewise, a putative sxtI encoding O-carbamoyltransferase (herein designated as AmsxtI) was recovered from the transcriptomic data, and its expression was investigated. The results revealed that the cellular toxin quota (Q) was higher in P-depleted, nitrate-grown cultures. With cultures at similar N:P (<16), cells grown with excess ammonium showed a higher Q than those grown with nitrate. sxtA1 was not expressed under any culture conditions, suggesting that this gene might not be involved in STX biosynthesis by this strain. Conversely, sxtA4 and sxtG showed positive correlations with Q, and were up-regulated in P-depleted, nitrate-grown cultures and with excess ambient ammonium. On the other hand, AmsxtI was expressed only when induced by P-depletion, suggesting that this gene may play an important role in P-recycling metabolism, while simultaneously enhancing toxin production.
石房蛤毒素（STXs）是一类强效的钠离子通道阻断毒素，也是麻痹性贝类毒素（PSP）的致病因子，由多种海洋甲藻和蓝藻产生。在亚历山大藻中，sxtA 和 sxtG 这两个 STX 核心基因已得到充分阐明，但在热带物种的各种营养模式下，这些基因的表达情况尚不清楚。本研究调查了在硝酸盐或铵盐中生长并具有不同氮磷（N:P）供应比的热带太平洋亚历山大藻的生理反应。观察了 sxt 基因的转录反应。同样，从转录组数据中回收了一个推定的 sxtI 编码 O-碳酰胺转移酶（在此命名为 AmsxtI），并对其表达进行了研究。结果表明，在磷饥饿、硝酸盐生长的培养物中，细胞毒素含量（Q）更高。在 N:P 相似（<16）的培养物中，与硝酸盐相比，在过量铵盐中生长的细胞表现出更高的 Q。在任何培养条件下都没有检测到 sxtA1 的表达，这表明该基因可能不参与该菌株的 STX 生物合成。相反，sxtA4 和 sxtG 与 Q 呈正相关，在磷饥饿、硝酸盐生长的培养物中和在过量环境铵盐中均上调表达。另一方面，只有在磷饥饿诱导时才表达 AmsxtI，这表明该基因可能在磷再循环代谢中发挥重要作用，同时增强毒素产生。
