Phylogenomic Analysis of the <i>Giardia intestinalis</i> Transcarboxylase Reveals Multiple Instances of Domain Fusion and Fission in the Evolution of Biotin-Dependent Enzymes

Sequencing of the gene encoding a pyruvate carboxylase-like protein from the amitochondrial eukaryote <i>Giardia</i><i>intestinalis</i> revealed a 1,338 aa protein composed of acetyl-CoA carboxyltransferase (ACCT), pyruvate carboxyltransferase (PycB), and biotin carboxyl carrier protein (BCCP) domains, linked in a single polypeptide chain. This particular domain combination has been previously seen only in the methylmalonyl-CoA:pyruvate transcarboxylase from <i>Propionibacterium</i><i>freudenreichii</i>, where each of these domains is encoded by an individual gene and forms a separate subunit. To get an insight into the evolutionary origin and biochemical function of the <i>G.</i><i>intestinalis</i> enzyme, we compared its domain composition to those of other biotin-dependent enzymes and performed a phylogenetic analysis of each of its domains. The results obtained indicate that: (1) evolution of the BCCP domain included several domain fusion events, leading to the ACCT-BCCP and PycB-BCCP domain combinations; (2) fusions of the PycB and BCCP domains in pyruvate carboxylases and oxaloacetate decarboxylases occurred on several independent occasions in different prokaryotic lineages, probably due to selective pressure towards co-expression of these genes, and (3) because newly sequenced biotin-dependent enzymes are often misannotated in sequence databases, their annotation as either carboxylases, decarboxylases, or transcarboxylases has to rely on detailed analysis of their domain composition, operon organization of the corresponding genes, gene content in the particular genome, and phylogenetic analysis.