Biosynthetic Intermediate Analysis and Functional Homology Reveal a Saxitoxin Gene Cluster in Cyanobacteria. Applied and environmental microbiology [Appl Environ Microbiol] Journal article

Saxitoxin (STX) and its analogues cause the paralytic shellfish poisoning (PSP) syndrome, which afflicts human health and impacts on coastal shellfish economies worldwide. PSP toxins are unique alkaloids, being produced by both prokaryotes and eukaryotes. Here we describe a candidate PSP toxin biosynthesis gene cluster (sxt) from Cylindrospermopsis raciborskii T3. The saxitoxin biosynthetic pathway is encoded by more than 35 kb and comparative sequence analysis assigns 30 catalytic functions to 26 proteins. STX biosynthesis is initiated with arginine, SAM and acetate by a new type of polyketide synthase, which can putatively perform a methylation of acetate, and a Claisen-condensation between propionate and arginine. Further steps involve enzymes catalyzing three heterocyclisations and various tailoring reactions that result in the numerous isoforms of saxitoxin. In the absence of a gene transfer system in these microorganisms, we have revised the known STX biosynthetic pathway, based on in silico functional inferences of sxt ORFs combined with LC-MS/MS analysis of the biosynthetic intermediates. Our results indicate the evolutionary origin for the production of PSP toxins in an ancestral cyanobacterium with genetic contributions from diverse phylogenetic lineages of bacteria, and provide a quantum addition to the catalytic collective available for future combinatorial biosyntheses. The distribution of these genes also supports the involvement of this gene cluster in STX production in various cyanobacteria.

Applied and environmental microbiology [Appl Environ Microbiol]