Molecular cloning, origin, and expression of saxitoxin biosynthesis gene sxtB from the toxic dinoflagellate Alexandrium catenella

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 degrees C (3.36-fold) and exposed to cold stress (20 -> 12 degrees C; 1.88-fold, 20 -> 16 degrees 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.

Automated summary

This study reports the characterization and transcriptional regulation of sxtB in the toxic dinoflagellate Alexandrium catenella, which produces STX. The gene is phylogenetically close to toxic cyanobacteria and its transcription levels are significantly up-regulated under low temperature conditions, showing a high correlation with increased STX contents.