In vivo characterization of the first acyl-CoA Δ6-desaturase from a member of the plant kingdom, the microalga Ostreococcus tauri

Genomic DNA of Ostreococcus tauri, a fully sequenced marine unicellular alga from the phytoplankton, was used to amplify a gene coding for a typical front-end desaturase involved in polyunsaturated fatty acid biosynthesis. Heterologous expression in Saccharomyces cerevisiae revealed very high desaturation activity with Delta(6)-regioselectivity. Short-time kinetic experiments showed that the desaturase product was detected in the acyl-CoA pool 5 min after addition of the exogenous substrate to the yeast medium and long before its appearance in the total fatty acids. When this desaturase was co-expressed with the acyl-CoA Delta(6)-elongase from Physcomitrella patens and the lipid-linked Delta(5)-desaturase from Phaeodactylum tricornutum, high proportions of arachidomic or eicosapentaenoic acid were obtained, because nearly all of the Delta(6)-desaturated products were elongated. Furthermore, the product/educt ratios calculated in each glycerolipid for the Delta(6)-desaturase or for the acyl-CoA Delta(6)-elongase were in about the same range, whereas this ratio showed a very uneven profile in the case of the lipid-linked Delta(5)-desaturase. Finally, a sequence-based comparison of all the functionally characterized Delta(6)-desaturases showed that this enzyme was not related to any previously described sequence. Altogether, our data suggest that this desaturase from O. tauri is an acyl-CoA Delta(6)-desaturase, the first one cloned from a photosynthetically active organism.