Heterologous Expression of an Unusual Ketosynthase, SxtA, Leads to Production of Saxitoxin Intermediates in Escherichia coli

Paralytic shellfish toxins (PSTs) are neurotoxic alkaloids produced by freshwater cyanobacteria and marine dinoflagellates. Due to their antagonism of voltage-gated sodium channels in excitable cells, certain analogues are of significant pharmacological interest. The biosynthesis of the parent compound, saxitoxin, is initiated with the formation of 4-amino-3-oxo-guanidinoheptane (ethyl ketone) by an unusual polyketide synthase-like enzyme, SxtA. We have heterologously expressed SxtA from Raphidiopsis raciborskii T3 in Escherichia coli and analysed its activity in vivo. Ethyl ketone and a truncated analogue, methyl ketone, were detected by HPLC-ESI-HRMS analysis, thus suggesting that SxtA has relaxed substrate specificity in vivo. The chemical structures of these products were further verified by tandem mass spectrometry and labelled-precursor feeding with [guanidino-15 N2 ] arginine and [1,2-13 C2 ] acetate. These results indicate that the reactions catalysed by SxtA could give rise to multiple PST variants, including analogues of ecological and pharmacological significance.