Systematics, Biodiversity and Evolution of Plants
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Thursday January 9th 2025, 11 am (CET) 7 pm (Japan Time)

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Y. Cho1, S. Hidema2, T. Omura3, K. Koike4, K. Koike5, S. Tsuchiya1, K. Konoki1, Y. Oshima6#, M. Yotsu-Yamashita1

Saxitoxin biosynthesis and metabolism in dinoflagellates as revealed by metabolic fluxes analysis and studies of early biosynthetic enzymes

1Graduate School of Agricultural Science, Tohoku University, Sendai, Japan
2Fukushima Medical University, Fukushima, Japan
3Tokyo University of Marine Science and Technology, Tokyo, Japan
4Graduate School of Integrated Sciences for Life, Hiroshima University, Higashi-Hiroshima, Japan
5Natural Science Center for Basic Research and Development Hiroshima University, Higashi-Hiroshima, Japan
6Graduate School of Life Sciences, Tohoku University, Sendai, Japan
# Proffesor emeritus
yuko.cho.a4@tohoku.ac.jp 

 

Saxitoxin (STX) and its analogues are collectively known as paralytic shellfish toxins (PSTs) and are causative agents of paralytic shellfish poisonings. Because these are potent inhibitors of voltage-dependent sodium channels, consuming shellfish contaminated with PSTs can cause food poisoning. The genuine producers of PSTs are various species of marine dinoflagellates (the genera Alexandrium, Pyrodinium and Gymnodinium). The harmful blooms of PST-producing dinoflagellates are found throughout the world and caused serious threats to human health. To develop strategies to predict changes in risk, it is important to elucidate the mechanisms of STX biosynthesis and metabolism. The STX biosynthesis gene clusters (a total of 21 genes) have been identified in another STX producing organism, cyanobacteria. Based on the putative function of these genes and identification of intermediates, the STX biosynthetic pathway was proposed1, 2. However, STX biosynthesis in dinoflagellates have not yet been fully elucidated, because of the unique characteristics of dinoflagellates, such as large genome size, high gene copy number and un-clustered arrangement of genes. Only five putative genes have thus far been reported as full-length sequences in dinoflagellates (sxtA, sxtG, sxtB, sxtI, and sxtU). We are trying to approach the issue from various aspects. By the isotope assisted metabolic flux analysis of STX related compounds (precursors, intermediates and STXs): in vivo labeling method, we proposed the hypothesis that STXs are biosynthesized through de novo and salvage biosynthesis3, 4. Furthermore, we have been focusing on the key enzymes, SxtA and SxtG, that catalyze early steps among STX biosynthetic enzymes. The analysis of abundance and localization of them revealed that SxtA and SxtG are expressed in chloroplasts and the absence of SxtA leads to loss of toxin producibility in the non-toxic subclone of Alexandrium catenella (Group I) 5, 6. The methods we have developed will be useful for elucidating the biosynthesis and metabolism of STX in dinoflagellates, for which conventional genetic engineering methods are not suitable.

 

References

1. Tsuchiya, S.; Cho, Y.; Yoshioka, R.; Konoki, K.; Nagasawa, K.; Oshima, Y.; Yotsu-Yamashita, M. Angew. Chem. Int. Ed. 2017, 56, 5327–5331.
2. Hakamada, M.; Tokairin, C.; Ishizuka, H.; Adachi, K.; Osawa, T.; Aonuma, S.; Hirozumi, R.; Tsuchiya, S.; Cho, Y.; Kudo, Y.; Konoki, K.; Oshima, Y.; Nagasawa, K.; Yotsu‐Yamashita, M. Chem. Euro. J. 2024, 30, e202304238.
3. Cho, Y.; Tsuchiya, S.; Omura, T.; Koike, K.; Oikawa, H.; Konoki, K.; Oshima, Y.; Yotsu-Yamashita, M. Sci Rep., 2019, 9, 3460.
4. Cho, Y.; Tsuchiya, S.; Omura, T.; Koike, K.; Konoki, K.; Oshima, Y.; Yotsu-Yamashita, M. Harmful Algae 2023, 122, 102372.
5. Cho, Y.; Hidema, S.; Omura, T.; Koike, K.; Koike, K.; Oikawa, H.; Konoki, K.; Oshima, Y.; Yotsu-Yamashita, M. Harmful Algae 2021, 101, 101972.
6. Cho, Y.; Hidema, S.; Omura, T.; Tsuchiya, S.; Konoki, K.; Oshima, Y.; Yotsu-Yamashita, M. Harmful Algae 2024, 139, 102723.