Annotation of genes involved in glycerolipid biosynthesis in Chlamydomonas reinhardtii:: Discovery of the betaine lipid synthase BTA1Cr

Lipid metabolism in flowering plants has been intensely studied, and knowledge regarding the identities of genes encoding components of the major fatty acid and membrane lipid biosynthetic pathways is very extensive. We now present an in silico analysis of fatty acid and glycerolipid metabolism in an algal model, enabled by the recent availability of expressed sequence tag and genomic sequences of Chlamydomonas reinhardtii. Genes encoding proteins involved in membrane biogenesis were predicted on the basis of similarity to proteins with confirmed functions and were organized so as to reconstruct the major pathways of glycerolipid synthesis in Chlamydomonas. This analysis accounts for the majority of genes predicted to encode enzymes involved in anabolic reactions of membrane lipid biosynthesis and compares and contrasts these pathways in Chlamydomonas and flowering plants. As an important result of the bioinformatics analysis, we identified and isolated the C. reinhardtii BTAI (BTA1(Cr)) gene and analyzed the bifunctional protein that it encodes; we predicted this protein to be sufficient for the synthesis of the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine(DGTS), a major membrane component in Chlamydomonas. Heterologous expression of BTA1(Cr), led to DGTS accumulation in Escherichia coli, which normally lacks this lipid, and allowed in vitro analysis of the enzymatic properties of BTA1(Cr). In contrast, in the bacterium Rhodobacter sphaeroides, two separate proteins, BtaA(Rs) and BtaB(Rs), are required for the biosynthesis of DGTS. Site-directed mutagenesis of the active sites of the two domains of BTA1(Cr) allowed us to study their activities separately, demonstrating directly their functional homology to the bacterial orthologs BtaA(Rs) and BtaB(Rs).