Brevetoxin: How Is It Made and Why
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Karenia brevis is the major harmful algal bloom-forming species in the Gulf of Mexico, and produces neurotoxins, known as brevetoxins, that cause large fish kills, neurotoxic shellfish poisoning, and human respiratory distress. Brevetoxins are polyethers that bind voltage-sensitive sodium channels, opening them for prolonged periods of time. Clonal cultures of K. brevis exhibit unique brevetoxin profiles, which not only differ from one another, but also change when subjected to different environmental conditions. The brevetoxin structures were elucidated 30 years ago without any breakthroughs for the biosynthetic pathway. These unique ladder-like polyethers have 10 (PbTx-1) or 11 (PbTx-2) rings, indicating that they are synthesized as secondary metabolites by polyketide synthases. The extensive size of the genome and the lack of histones and nucleosomes combined with the additional regulatory step of a trans-splicing spliced leader sequence make normal molecular techniques ineffective in determining the genes involved in toxin synthesis. The goal of this project is to identify a potential link between toxin, gene, and function. One objective is to take the next step towards identifying the genes associated with the synthesis and regulation of brevetoxins and to help elucidate the hypothesized gene clusters of multi-protein enzymatic complexes involved in brevetoxin production, one for each backbone. The second objective is to make an effort to determine the in vivo function of the costly brevetoxins by identifying possible ion channels, which could be osmotically regulated by the toxins. Genes for polyketide synthases (PKS) were identified in K. brevis, obtained from Expressed Sequence Tag (EST) libraries. In this work, reverse transcription polymerase chain reactions (RT-PCR) were used to generate pools of complementary DNA (cDNA), which was used in real-time quantitative polymerase chain reactions (qPCR) to give relative amounts of PKS transcripts. K. brevis clones have shown a significant increase in toxin production after a rapid shift from high salinity to low salinity, indicating a regulation of brevetoxin synthesis. To gain a better understanding of regulation of toxin production during algal blooms, we compared the toxin levels under different conditions to the transcript levels of PKS genes, as determined by quantitative RT-PCR. In a separate line of investigation, an in silico analysis of the EST library was performed to identify ion channel genes expressed by K. brevis, which may be the in vivo binding site of brevetoxin. The information generated from this project will help to elucidate the effects of environmental variations on toxin production and the biological function of toxin production -- valuable information for the shellfish industries and public health.
Thompson, Natalie (2011). Brevetoxin: How Is It Made and Why. Master's thesis, Texas A&M University. Available electronically from