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The gene cluster is characterized for the biosynthesis of the cyanobacterial toxin cylindrospermopsin (cyr) in Cylindrospermopsis raciborskii AWT205 and the complete biosynthetic pathway is described. The results have helped in the design of toxic strain-specific probes and also for the study of the regulation and biological role of cylindrospermopsin.

We report the first characterization of anl-arginine:glycine amidinotransferase from a prokaryote. The enzyme, CyrA, is involved in the pathway for biosynthesis of the polyketide-derived hepatotoxin cylindrospermopsin from Cylindrospermopsis raciborskii AWT205. CyrA is phylogenetically distinct from other amidinotransferases, and structural alignment shows differences between the active site residues of CyrA and the well-characterized humanl-arginine:glycine amidinotransferase (AGAT). Overexpression of recombinant CyrA in Escherichia coli enabled biochemical characterization of the enzyme, and we confirmed the predicted function of CyrA as anl-arginine:glycine amidinotransferase by 1H NMR. As compared with AGAT, CyrA showed narrow substrate specificity when presented with substrate analogs, and deviated from regular Michaelis–Menten kinetics in the presence of the non-natural substrate hydroxylamine. Studies of initial reaction velocities and product inhibition, and identification of intermediate reaction products, were used to probe the kinetic mechanism of CyrA, which is best described as a hybrid of ping-pong and sequential mechanisms. Differences in the active site residues of CyrA and AGAT are discussed in relation to the different properties of both enzymes. The enzyme had maximum activity and maximum stability at pH 8.5 and 6.5, respectively, and an optimum temperature of 32 °C. Investigations into the stability of the enzyme revealed that an inactivated form of this enzyme retained an appreciable amount of secondary structure elements even on heating to 94 °C, but lost its tertiary structure at low temperature ( Tmax of 44.5 °C), resulting in a state reminiscent of a molten globule. CyrA represents a novel group of prokaryotic amidinotransferases that utilize arginine and glycine as substrates with a complex kinetic mechanism and substrate specificity that differs from that of the eukaryoticl-arginine:glycine amidinotransferases. [ABSTRACT FROM AUTHOR]

Background: Dolichospermum circinale is a filamentous bloom-forming cyanobacterium responsible for biosynthesis of the paralytic shellfish toxins (PST), including saxitoxin. PSTs are neurotoxins and in their purified form are important analytical standards for monitoring the quality of water and seafood and biomedical research tools for studying neuronal sodium channels. More recently, PSTs have been recognised for their utility as local anaesthetics. Characterisation of the transcriptional elements within the saxitoxin (sxt) biosynthetic gene cluster (BGC) is a first step towards accessing these molecules for biotechnology., Results: In D. circinale AWQC131C the sxt BGC is transcribed from two bidirectional promoter regions encoding five individual promoters. These promoters were identified experimentally using 5' RACE and their activity assessed via coupling to a lux reporter system in E. coli and Synechocystis sp. PCC 6803. Transcription of the predicted drug/metabolite transporter (DMT) encoded by sxtPER was found to initiate from two promoters, PsxtPER1 and PsxtPER2. In E. coli, strong expression of lux from PsxtP, PsxtD and PsxtPER1 was observed while expression from Porf24 and PsxtPER2 was remarkably weaker. In contrast, heterologous expression in Synechocystis sp. PCC 6803 showed that expression of lux from PsxtP, PsxtPER1, and Porf24 promoters was statistically higher compared to the non-promoter control, while PsxtD showed poor activity under the described conditions., Conclusions: Both of the heterologous hosts investigated in this study exhibited high expression levels from three of the five sxt promoters. These results indicate that the majority of the native sxt promoters appear active in different heterologous hosts, simplifying initial cloning efforts. Therefore, heterologous expression of the sxt BGC in either E. coli or Synechocystis could be a viable first option for producing PSTs for industrial or biomedical purposes.