Pathways for phosphatidylcholine biosynthesis in bacteria

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Abstract
Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes with important structural and signalling functions. Although many prokaryotes lack PC, it can be found in significant amounts in membranes of rather diverse bacteria. Two pathways for PC biosynthesis are known in bacteria, the methylation pathway and the phosphatidylcholine synthase (PCS) pathway. In the methylation pathway, phosphatidylethanolamine is methylated three times to yield PC, in reactions catalysed by one or several phospholipidN-methyltransferases (PMTs). In the PCS pathway, choline is condensed directly with CDP-diacylglyceride to form PC in a reaction catalysed by PCS. Using cell-free extracts, it was demonstrated thatSinorhizobium meliloti,Agrobacterium tumefaciens,Rhizobium leguminosarum,Bradyrhizobium japonicum,Mesorhizobium lotiandLegionella pneumophilahave both PMT and PCS activities. In addition,Rhodobacter sphaeroideshas PMT activity andBrucella melitensis,Pseudomonas aeruginosaandBorrelia burgdorferihave PCS activities. Genes fromM. lotiandL. pneumophilaencoding a Pmt or a Pcs activity and the genes fromP. aeruginosaandBorrelia burgdorferiresponsible for Pcs activity have been identified. Based on these functional assignments and on genomic data, one might predict that if bacteria contain PC as a membrane lipid, they usually possess both bacterial pathways for PC biosynthesis. However, important pathogens such asBrucella melitensis,P. aeruginosaandBorrelia burgdorferiseem to be exceptional as they possess only the PCS pathway for PC formation.