Prokaryotic origin of cytidylyltransferases and α-ketoacid synthases

α-Ketoacids are present in glycosylated structures in almost all organisms and must be activated by a cytidylyltransferase (CT) before their incorporation into glycoconjugates. Examples of α-ketoacids include KDO (keto-deoxyoctulosonic acid), which is present in bacterial lipopolysaccharide and in plant pectins, and sialic acids (Sia), such as N-acetylneuraminate (NeuAc), which are present in animals and in pathogenic microorganisms. The phylogeny of Sia and CTs is unclear but is linked to the history of the α-ketoacid synthases. Furthermore, horizontal gene transfer (HGT) events might have played a major role. Here we analyse the origin and the expansion process of these genes with respect to the taxonomic coherence of the phylogenetic trees, the molecular characteristics of the CT-coding DNA and the presence or absence of a long C-terminal coding region in some NeuAc-CTs. We propose a prokaryotic origin for CTs and α-ketoacid synthases, and a HGT event of these genes towards ancestors of animals and plants. Finally, some pathogenic bacteria reacquired some of these genes, which would have been modified and devoted to Sia synthesis.