Polyketide synthase genes from marine dinoflagellates.

Rapidly developing techniques for manipulating the pathways of polyketide biosynthesis at the genomic level have created the demand for new pathways with novel biosynthetic capability. Polyketides derived from dinoflagellates are among the most complex and unique structures identified thus far, yet no studies of the biosynthesis of dinoflagellate-derived polyketides at the genomic level have been reported. Nine strains representing 7 different species of dinoflagellates were screened for the presence of type I and type II polyketide synthases (PKSs) by polymerase chain reaction (PCR) and reverse transcriptase PCR. Seven of the 9 strains yielded products that were homologous with known and putative type I PKSs. Unexpectedly, a PKS gene was amplified from cultures of the dinoflagellate Gymnodinium catenatum, a saxitoxin producer, which is not known to produce a polyketide. In each case the presence of a PKS gene was correlated with the presence of bacteria in the cultures as identified by amplification of the bacterial 16S ribosomal RNA gene. However, amplification from polyadenylated RNA, the lack of PKS expression in light-deprived cultures, residual phylogenetic signals, resistance to methylation-sensitive restriction enzymes, and the lack of hybridization to bacterial isolates support a dinoflagellate origin for most of these genes.

Are Pfiesteria species toxicogenic? Evidence against production of ichthyotoxins by Pfiesteria shumwayae.

The estuarine genus Pfiesteria has received considerable attention since it was first identified and proposed to be the causative agent of fish kills along the mid-Atlantic coast in 1992. The presumption has been that the mechanism of fish death is by release of one or more toxins by the dinoflagellate. In this report, we challenge the notion that Pfiesteria species produce ichthyotoxins. Specifically, we show that (i) simple centrifugation, with and without ultrasonication, is sufficient to "detoxify" water of actively fish-killing cultures of Pfiesteria shumwayae, (ii) organic extracts of lyophilized cultures are not toxic to fish, (iii) degenerate primers that amplify PKS genes from several polyketide-producing dinoflagellates failed to yield a product with P. shumwayae DNA or cDNA, and (iv) degenerate primers for NRPS genes failed to amplify any NRPS genes but (unexpectedly) yielded a band (among several) that corresponded to known or putative PKSs and fatty acid synthases. We conclude that P. shumwayae is able to kill fish by means other than releasing a toxin into bulk water. Alternative explanations of the effects attributed to Pfiesteria are suggested.