Intracellular abundance, localization, and enzymatic activity of a saxitoxin biosynthesis enzyme, SxtG, in two sister subclones of the dinoflagellate Alexandrium catenella with extremely different levels of paralytic shellfish toxins

•Saxitoxin biosynthetic enzyme, SxtG, localizes to chloroplasts.•The PST-negative subclone of Alexandrium catenella expresses active SxtG but not SxtA.•The first in vitro reproduction of the early step of STX biosynthesis in dinoflagellates. Paralytic shellfish poisoning is caused by saxitoxin (STX), and its analogues (paralytic shellfish toxins (PSTs)) produced by marine dinoflagellates. SxtA and SxtG are the most essential enzymes in STX biosynthesis. Previous studies investigated the abundance and subcellular localization (i.e., chloroplasts) of SxtA in dinoflagellates using immunostaining. The present study characterized SxtG, and positive signals were detected in sister subclones of Alexandrium catenella (Group I) with extremely different levels of PSTs. Multiplex fluorescence immunostaining detection of a PST-positive subclone revealed co-localization of SxtA and SxtG, suggesting that SxtG localizes to chloroplasts. In vitro amidino-transfer from arginine to Int-A’, the first intermediate product in the biosynthesis, was presumed to be catalyzed by SxtG, and the reaction was established using crude extracts of PST-positive and negative A. catenella subclones. These analyses suggested that the PST-negative subclone expresses active SxtG but not SxtA. These findings support our hypothesis that decrease of SxtA leads to the loss of toxicity in the PST-negative subclone of A. catenella. Our results identified a key reaction that could enhance understanding of the biochemistry of STX biosynthesis in dinoflagellates. [Display omitted]