Author: Alescia Cullen
A major reason cyanobacterial blooms are of nuisance is due to their ability to produce toxic compounds. One class of these compounds are the Paralytic Shellfish Toxins (PSTs), which encompass saxitoxin and its related analogues. Despite the identification of the putative genetics underlying biosynthesis of saxitoxin in 2008, biochemical confirmation of its products and analogues remains mostly uncharacterized. This study investigated two PST-producing strains of Scytonema crispum, CAWBG524 and CAWBG72, isolated in New Zealand. Genome sequencing of both strains revealed the largest sxt clusters described, with the highest abundance in transposases. While these two strains show high genetic similarity, their toxin profiles vastly vary. S. cripsum CAWBG524 only produces saxitoxin while S. crispum CAWBG72 produces a range of saxitoxin analogues that are sulphated at numerous positions. Further genome analysis correlated these variations to inactivating transposase insertions into genes encoding tailoring enzymes sxtN, sxtO and sxtDIOX, in S. crispum CAWBG524. These enzymes were predicted to be responsible for the production of sulphated analogues. To validate this hypothesis, we investigated the function of putative sulphotransferase SxtN via binding to proposed substrates and in vitro biochemical assays. Our results confirmed that SxtN acts as a N-sulfotransferase on the N21 position of saxitoxin yielding the monosulphated analogue gonyautoxin-5, thus explaining the lack of analogues in S. crispum CAWBG524. This is the first biochemical characterization of the biosynthesis of saxitoxin analogues using a putative sxt cluster.