br Enzymology of TAG biosynthesis br

Enzymology of TAG biosynthesis
In search of algal DGATs
Discussion It has been assumed that algal fatty CRT 0066101 and TAG biosynthesis would be similar to that of higher plants, and indeed in silico analysis reveals many sequences in algal genomes that are likely to encode the relevant enzymes for fatty acid biosynthesis and TAG biosynthesis (Hu et al., 2008, Miller et al., 2010). However, it is becoming clear that for most algal species, even for those that fall into the green algal lineage, there are some notable differences. With regard to the DGATs, we have found that the majority of algal species encode at least one DGAT1, although it appears to be absent from Ostreococcus and Micromonas, as is the case for yeast, S. cerevisiae. More surprising is the presence of multiple DGAT2 genes in algae, with the exception of C. merolae, which has one gene, and E. huxleyi, which has none (Table 2). If the latter observation is confirmed to be true, this will be a novel finding for eukaryotes. Nevertheless in all other algae surveyed, 2 or more DGAT2 genes were found. An interesting pattern that emerged from comparing the sequences of these multiple DGAT2s was that the genes in a particular algal species are more divergent from each other than they are between algal species. Green and red algae are thought to have arisen by the endosymbiosis of a cyanobacterium that gave rise to the chloroplast. All land plants arose from the charophyte branch of the green algal lineage. Diatoms, heterokonts and haptophytes are much more divergent from these basal algae, and from each other, although they all contain complex chloroplasts, which arose through secondary endosymbiosis of a red alga (Dorrell and Smith, 2011). We therefore expected that green algal DGATs should be most related in terms of sequence similarities to higher plants, followed by red algae and other algae clades. From the dendogram of DGAT2s, it is clear that this is not the case: the multiple DGAT2s within an algal species are highly divergent, more so than the DGAT2s between different higher plant species or between mammalian and fungal DGAT2s. The vast majority of algal DGAT2s seem to be distantly related to both higher plant and animal DGAT2s. Assuming that biochemical analysis confirms the enzymatic identity of these putative DGAT2s, we tentatively propose that the various putative DGAT2 isoforms found in modern algal groups could represent a very ancient gene duplication event that occurred prior to the subsequent divergence of various eukaryotic lineages. These isoforms were then gradually lost in eukaryotic lineages that would form the basal groups of complex multicellular organisms until only one particular isoform was selected for prior to the speciation of multicellular organisms. The algal lineages that did not develop complex multicellularity retained the various DGAT2 isoforms, perhaps to enable the production of different TAGs within the same cell, which in multicellular organisms can take place via spatial separation of fatty acid biosynthesis in different cell types. A good way to validate this theory would be to look for DGAT2s in basal lineages of eukaryotes as well as multicellular brown algae to see if this pattern can be seen across the tree of life.