The 12 th Symposium of the Malaysian Society of Applied Biology 2012 Applied Biology: Solutions to Global Challenges and Issues 196 ORAL INFO035 Characterization of the Saxitoxin Biosynthetic Starting Gene, sxta in the Toxic Dinoflagellate Alexandrium tamiyavanichii KIENG-SOON HII 1 , PO-TEEN LIM 2 , TOH-HII TAN 1 AND CHUI-PIN LEAW 1 * 1 Institute of Biodiversity and Environmental Conservation, University Malaysia Sarawak, Kota Samarahan, 94300 Sarawak, Malaysia 2 Faculty of Resource Science and Technology, Universiti Malaysia Sarawak, Kota Samarahan, 94300 Sarawak, Malaysia *Corresponding e-mail: cpleaw@ibec.unimas.my ABSTRACT Recently, molecular genetics of the Saxitoxin (STX) biosynthesis pathway has become one of the major focuses in paralytic shellfish poisoning (PSP) toxin-related studies after the recent discovery of STX biosynthetic genes in toxic cyanobacteria and later in the toxic dinoflagellates. Here we described the two domains of sxtA, SAM-dependent methyltransferase coding gene, sxtA1 and the class II aminotransferase coding gene, sxtA4 from a toxic strain of Alexandrium tamiyavanichii isolated from Samariang, Sarawak. The partial coding sequences of sxtA1 and sxtA4 were 432 bp and 639 bp respectively, with the deduced amino acid sequences revealed polypeptides of 144 and 213 amino acid residues, respectively. The present results showed high sequence similarity and identity (~91% and ~98%, respectively) compared to other PSP toxins-producing dinoflagellates. Indeed, our protein phylogenetic analyses revealed close relationship of both A. tamiyavanichii sxtA1 and sxtA4 to others PSP toxins-producing dinoflagellates, with sxtA from PSP toxins-producing cyanobacteria and putative toxin-related genes forming the sister clades. Keywords: saxitoxin, paralytic shellfish poisoning, SAM-dependent methyltransferase (sxtA1), class II aminotransferase (sxtA4), Alexandium tamiyavanichii. INTRODUCTION Saxitoxin (STX) is a highly potent neurotoxin that caused paralytic shellfish poisoning (PSP), which is an affliction to human health and coastal shellfish industries. In the marine environment, the eukaryotic dinoflagelates in the genera of Alexandrium, Pyrodinium and Gymnodinium are the major species that capable of producing STXs. While in the freshwater ecosystem, some prokaryotic cyanobacteria such as Anabaena circinalis, Aphanizomenon sp. Cylidrospermopsis raciborskii and Lyngbya wollei are the major STXs producers. Production of STX and its derivatives, involves a very complicated biosynthesis pathway. The first saxitoxin biosynthesis pathway was elucidated base on precursor-feeding experiments (Shimizu et al., 1984). And two decades later, the saxitoxin biosynthesis genes (sxt) were discovered from cyanobacteria by in-silico functional homology analysis and LC- MS analysis of the biosynthesis intermediates (Kellmann et al., 2008a; Kellmann et al.,