Omega-6-fatty-acid desaturase

Omega-6-fatty-acid desaturase

Korn and Greenblatt (Korn, E.D. & Greenblatt, C.L. Synthesis of alpha-linolenic acid by Leishmania enriettii. Science 142, 1301-1303 (1963)) have reported that Leishmania enriettii is capable of synthesizing the poly-unsaturated fatty acids linoleic acid and alpha-linolenic acid. This capability is generally restricted to higher plants and phytoflagellates such as Euglena. The enzymes implicated in the formation of these unsaturated fatty acids are omega-3 and omega-6 fatty acid desaturases, the genes of which have so far only been described for plants. When we searched the T. brucei genome database we found a gene fragment with significant similarity to the endoplasmic reticulum isoenzyme of omega-6 fatty acid desaturase of soybean.

Further details of the analysis

A complete T. brucei desaturase gene was found in the trypanosome database (AC007862: 100883-102106). the ORF was translated and compared with the SwissProt database indexed at European Bioinformatics Institute (EBI, Hinxton UK) on 28 July 2001 and having 99162 entries, using the NCBI BLASTP program and the BLOSUM 62 matrix (http://www2.ebi.ac.uk/blastall/). Click here to inspect the BLASTP output file.

Interestingly, from the relevant scores all homologous sequences belonged to plants and cyanobacteria. No homologues from other organisms were found. Twenty-three desaturase sequences with significant E values (3e-78 and 3e-17) were selected and aligned using the "RunDBClustalW" option in the BLASTP output. The ClustalW alignment is availabe here for inspection. From this alignment positions with gaps were removed and the alignment in Phylip format containing 23 sequences and 331 positions was used to calculate pairwise distances between the sequences. Distances with the T. brucei desaturase ranged from 58-80%. The alignment, when freed from gaps, clearly showed three classes of sequence: the chloroplast and endoplasmic reticular omega-6 desaturases (comprising also the desaturases from the cyanobacteria Synechocystis and Spirulina platensis and that from T. brucei) and the omega-3 desaturases. This alignment was used for likelihood mapping, which revealed a high quality dataset with a good phylogenetic signal: only 3.1% star-like quartets and 94.5% of the quartets well resolved. This set was used for the calculation of neighbor-joining, maximum likelihood and maximal parsimony trees. The three trees were robust and displayed identical topologies. In all cases the T. brucei desaturase clustered with the plant omega-6 desaturases of the endoplamic reticulum with high bootstrap support or quartet frequency (ML ,100%; NJ, 99%; MP, 96%).

Conclusion

The T. bucei desaturase clusters robustly with the plant omega-6 desaturases of the endoplamic reticulum. Since there are, so far, no desaturases found in other eukaryotes, this must be seen as a strong indication for an event of horizonta transfer from an alga or plant to a trypanosomatid.