Transcriptome sequencing reveals single domain Type I-like polyketide synthases in the toxic dinoflagellate Gambierdiscus polynesiensis

Abstract

Ciguatoxins (CTXs) are potent neurotoxins responsible for the food-borne illness known as ciguatera that occurs after consumption of contaminated fish. Benthic dinoflagellates of the genus Gambierdiscus spp. are known as the main producers of CTXs. CTXs are polycyclic polyethers, presumed to be synthesized by polyketide synthase (PKS) complexes; however, the mechanisms of CTX biosynthesis remain unresolved. Here, we investigated a de novo transcriptome assembly of Gambierdiscus polynesiensis TB-92 clone, a highly toxic producer of Pacific ciguatoxins, and focused on the identification of PKS transcripts. A cDNA library generated using a spliced leader (SL) priming approach, which specifically targets the dinoflagellate nuclear transcriptome, was sequenced by Roche 454. This strategy produced 1,221,335 raw reads, assembled into 16,336 unique contigs. Contigs were subjected to BLAST search, annotated with Gene Ontology (GO) terms and enriched with enzyme codes (EC) from Kyoto Encyclopedia of Genes and Genomes (KEGG) database. Thirty-three PKS-related sequences were thus identified. Twenty-two contigs encoded single domain β-ketoacyl synthases (KS) with sequence similarity to Type I PKSs, as reported in other dinoflagellates. A conserved motif previously observed near the 5′ end of dinoflagellate KS domain transcripts was present in G. polynesiensis, and distinguished two groups of KS domain sequences. Ketoreductase (KR), acyltransferase (AT), and acyl carrier protein (ACP) domains were also found on single domain containing transcripts. KEGG pathway mapping placed three of the KS sequences containing the PKS conserved domain (cd00833) in the fatty acid biosynthesis pathway. No contigs were found encoding the conserved domains typically found in elongating ketosynthase domains of fatty acid synthases (cd00832, Type I or cd00834, Type II). Contigs mapping to other parts of the fatty acid biosynthesis pathway similarly encoded individual domains, suggesting that fatty acid synthesis takes place in multiprotein complexes. Other than the three KS domains, none of the sequences mapping to the fatty acid biosynthesis pathway overlapped with those annotated as PKSs. These data lend support to the idea that PKSs may contribute to both polyketide and fatty acid synthesis in dinoflagellates. This dataset provides important background to future research in order to understand the complex mechanism of toxin production in this dinoflagellate.