Promoter regions of sxtA and sxtG reveal relationship between saxitoxin biosynthesis and photosynthesis in toxic Alexandrium catenella

The marine dinoflagellate Alexandrium catenella (Group I) produces saxitoxins (STXs) in marine environments, causing paralytic shellfish poisoning (PSP). The toxins are synthesized by STXs biosynthesis genes (sxt); of them, sxtA and sxtG are considered the most essential. The promoter region and 3'-untranslated region (3'-UTR) are known to regulate gene transcription; however, they have not been sufficiently elucidated in the sxt genes. Herein, we determined and characterized the genomic DNA sequence of the proximal regions of sxtA and sxtG in A. catenella. The promoter regions of sxtA and sxtG in A. catenella were found to contained distinct cis-regulatory elements (CREs), CpG islands, and TTTT-motif (instead of TATA-box), showing a typical eukaryotic promoter. Especially, a number of light-responsive CREs (G-box, SORLIP1AT/2AT, and Sp1) were found, and the gene expression level showed a significant relationship with photosynthesis. MiSeq sequencing of the long PCR amplicons revealed that sxtA was encoded as a single exonic structure that is discrete (> 5 kbp) from other core sxt. In addition, we reported for the first time two types of 3'-UTR in sxtG mRNA, and their lengths and sequences varied among strains and species. Such results suggest that sxt may be regulated by post-transcription. Our results provide insights into the toxin dynamics of toxic dinoflagellates from a molecular perspective.