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      In Silico Analysis of Putative Paralytic Shellfish Poisoning Toxins Export Proteins in Cyanobacteria


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          Paralytic shellfish poisoning toxins (PSTs) are a family of more than 30 natural alkaloids synthesized by dinoflagellates and cyanobacteria whose toxicity in animals is mediated by voltage-gated Na + channel blocking. The export of PST analogues may be through SxtF and SxtM, two putative MATE (multidrug and toxic compound extrusion) family transporters encoded in PSTs biosynthetic gene cluster ( sxt). sxtM is present in every sxt cluster analyzed; however, sxtF is only present in the Cylindrospermopsis-Raphidiopsis clade. These transporters are energetically coupled with an electrochemical gradient of proton (H +) or sodium (Na +) ions across membranes. Because the functional role of PSTs remains unknown and methods for genetic manipulation in PST-producing organisms have not yet been developed, protein structure analyses will allow us to understand their function. By analyzing the sxt cluster of eight PST-producing cyanobacteria, we found no correlation between the presence of sxtF or sxtM and a specific PSTs profile. Phylogenetic analyses of SxtF/M showed a high conservation of SxtF in the Cylindrospermopsis-Raphidiopsis clade, suggesting conserved substrate affinity. Two domains involved in Na + and drug recognition from NorM proteins (MATE family) of Vibrio parahaemolyticus and V. cholerae are present in SxtF/M. The Na + recognition domain was conserved in both SxtF/M, indicating that Na + can maintain the role as a cation anti-transporter. Consensus motifs for toxin binding differed between SxtF and SxtM implying differential substrate binding. Through protein modeling and docking analysis, we found that there is no marked affinity between the recognition domain and a specific PST analogue. This agrees with our previous results of PST export in R. brookii D9, where we observed that the response to Na + incubation was similar to different analogues. These results reassert the hypothesis regarding the involvement of Na + in toxin export, as well as the motifs L 398XGLQD 403 (SxtM) and L 390VGLRD 395 (SxtF) in toxin recognition.

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              Biosynthetic intermediate analysis and functional homology reveal a saxitoxin gene cluster in cyanobacteria.

              Saxitoxin (STX) and its analogues cause the paralytic shellfish poisoning (PSP) syndrome, which afflicts human health and impacts 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, S-adenosylmethionine, and acetate by a new type of polyketide synthase, which can putatively perform a methylation of acetate, and a Claisen condensation reaction between propionate and arginine. Further steps involve enzymes catalyzing three heterocyclizations 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 description of the known STX biosynthetic pathway, with in silico functional inferences based on sxt open reading frames combined with liquid chromatography-tandem mass spectrometry 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 idea of the involvement of this gene cluster in STX production in various cyanobacteria.

                Author and article information

                Role: Editor
                PLoS One
                PLoS ONE
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                15 February 2013
                : 8
                : 2
                : e55664
                [1 ]Pontificia Universidad Católica de Chile, Santiago, Chile
                [2 ]Nanobiotechnology Division at University of Talca, Fraunhofer Chile Research Foundation - Center for Systems Biotechnology, Talca, Chile
                [3 ]Institute of Molecular Evolution Heinrich-Heine, Universität Düsseldorf, Düsseldorf, Germany
                [4 ]Universidad Andres Bello, Center for Bioinformatics and Integrative Biology, Santiago, Chile
                Tel Aviv University, Israel
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: KSL XL FGN MV. Performed the experiments: KSL XL JJF KS. Analyzed the data: KSL XL JJF KS FGN MV. Contributed reagents/materials/analysis tools: FGN MV. Wrote the paper: KSL XL JJF KS MV.

                Copyright @ 2013

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                : 21 September 2012
                : 29 December 2012
                Page count
                Pages: 10
                The first author KSL is a Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) PhD fellow. This work was financed and supported by Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT) Grant 1050433 and 1080075. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Research Article
                Computational Biology
                Macromolecular Structure Analysis
                Protein Structure
                Molecular Genetics
                Gene Identification and Analysis
                Evolutionary Biology
                Evolutionary Systematics
                Marine Biology
                Freshwater Ecology
                Microbial Ecology
                Microbial Evolution
                Microbial Metabolism



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