pubmed.ncbi.nlm.nih.gov

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

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

Ralf Kellmann et al. Appl Environ Microbiol. 2008 Jul.

Abstract

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.

PubMed Disclaimer

Figures

FIG. 1.
FIG. 1.

Hypothetical biosynthesis pathway of STX as proposed by Shimizu et al. (47). Hypothetical intermediate metabolites are labeled with letters in brackets. The reaction steps are as follows: 1, Claisen condensation reaction between acetate and arginine; 2, amidino transfer from a second arginine to the α-amino group of intermediate B; 3, cyclization; 4, introduction of SAM methyl-derived side chain, involving the loss of one methionine methyl hydride; 5, epoxidation of side chain, leading to a 1,2-H shift; 6, opening of epoxide to an aldehyde followed by reduction of the aldehyde; 7 and 8, carbamoyl transfer and dihydroxylation.

FIG. 2.
FIG. 2.

Structural organization of the sxt gene cluster from C. raciborskii T3. Abbreviations used are as follows: IS4, insertion sequence 4; ompR, transcriptional regulator of ompR family; hisA, two-component histidine kinase; orf24, ORF 4. The scale indicates the gene cluster lengths in base pairs.

FIG. 3.
FIG. 3.

Revised pathway for STX biosynthesis and the putative functions of sxt genes (see text for description).

FIG. 4.
FIG. 4.

MS-MS spectra of selected ions from cellular extracts of C. raciborskii T3. The predicted fragmentation of ions and the corresponding m/z values are indicated. a, arginine (m/z 175); b, saxitoxin (m/z 300) (refer to Sleno et al. [51] for a more detailed identification and discussion of the fragment ions obtained from the CID of saxitoxin); c, intermediate A′ (m/z 187); d, intermediate C′ (m/z 211); e, intermediate E′ (m/z 225).

Similar articles

Cited by

References

    1. Akoh, C. C., G. C. Lee, Y. C. Liaw, T. H. Huang, and J. F. Shaw. 2004. GDSL family of serine esterases/lipases. Prog. Lipid Res. 43:534-552. - PubMed
    1. Alexander, F. W., E. Sandmeier, P. K. Mehta, and P. Christen. 1994. Evolutionary relationships among pyridoxal-5′-phosphate-dependent enzymes. Regio-specific alpha, beta and gamma families. Eur. J. Biochem. 219:953-960. - PubMed
    1. Anderson, D. M., D. M. Kulis, J. J. Sullivan, S. Hall, and C. Lee. 1990. Dynamics and physiology of saxitoxin production by the dinoflagellates Alexandrium spp. Mar. Biol. 104:511-524.
    1. Arakawa, O., S. Nishio, T. Noguchi, Y. Shida, and Y. Onoue. 1995. A new saxitoxin analogue from a xanthid crab Atergatis floridus. Toxicon 33:1577-1584. - PubMed
    1. Arakawa, O., T. Noguchi, Y. Shida, and Y. Onoue. 1994. Occurrence of carbamoyl-N-hydroxy derivatives of saxitoxin and neosaxitoxin in a xanthid crab Zosimus aeneus. Toxicon 32:175-183. - PubMed

Publication types

MeSH terms

Substances

LinkOut - more resources