Saxitoxin (STX) is a neurotoxin produced by certain toxic cyanobacteria and dinoflagellates. It is biochemically synthesized by the stepwise involvement of eight core enzymes encoded in sxt genes. Of them, sxtB, a cytidine deaminase (CDA)-like enzyme, may participate in the third step (after sxtA and sxtG) in the toxin synthesis pathway; however, it is insufficiently elucidated in toxic dinoflagellates. In the present study we determined a novel sxtB gene from the STX-producing dinoflagellate Alexandrium catenella, characterized structural motifs and phylogenetic origin, and evaluated transcriptional responses of the gene under different temperatures. cDNA of the AcsxtB was 1,146 bp in length from dinoflagellate spliced leader (Dino-SL) to poly (A) tail. It comprised a 972 bp open reading frame, encoding a 323 aa protein with a molecular weight of 34.20 kDa and isoelectric point of 6.36. In addition, we identified a putative mitochondrial transfer peptide and no introns in genomic coding regions. The AcsxtB was phylogenetically close to toxic cyanobacteria, but distant to non-toxic dinoflagellates. The transcription levels of AcsxtB were significantly up-regulated when cultured at 16 °C (3.36-fold) and exposed to cold stress (20 → 12 °C; 1.88-fold, 20 → 16 °C; 2.07-fold), which showed a high correlation with increased STX contents. These suggest that the sxtB should participate in STX synthesis, but its involvement may be low or regulated in very early stages before 72 h. This is the first report on sxtB characterization and transcriptional regulation with relevance to STX production in the toxic dinoflagellates.

石房蛤毒素 (STX) 是一种由某些有毒蓝藻和甲藻产生的神经毒素。它是通过逐步参与sxt基因中编码的八种核心酶进行生化合成的。其中，sxtB是一种胞苷脱氨酶 (CDA) 样酶，可能参与毒素合成途径的第三步（在sxtA和sxtG之后）；然而，它在有毒的甲藻中没有得到充分阐明。在本研究中，我们从产生 STX 的甲藻链状亚历山大藻中确定了一个新的sxtB基因，表征了结构基序和系统发育起源，并评估了该基因在不同温度下的转录反应。AcsxtB的 cDNA从甲藻剪接前导 (Dino-SL) 到聚 (A) 尾的长度为 1,146 bp。它包含一个 972 bp 的开放阅读框，编码一个 323 个氨基酸的蛋白质，分子量为 34.20 kDa，等电点为 6.36。此外，我们确定了一种假定的线粒体转移肽，并且在基因组编码区中没有内含子。AcsxtB 在系统发育上接近有毒蓝藻，但远离无毒甲藻。AcsxtB的转录水平在 16 °C（3.36 倍）培养和暴露于冷应激（20 → 12 °C；1.88 倍，20 → 16 °C；2.07 倍）时显着上调，这表明与增加的 STX 含量高度相关。这些表明sxtB应该参与 STX 合成，但其参与可能很低或在 72 小时之前的早期阶段受到调节。这是关于与有毒甲藻中 STX 产生相关的sxtB表征和转录​​调控的第一份报告。