Characterization of a New Type of Phosphopantetheinyl Transferase for Fatty Acid and Siderophore Synthesis in Pseudomonas aeruginosa

Phosphopantetheinyl-dependent carrier proteins are part of fatty-acid synthases (primary metabolism), polyketide synthases, and non-ribosomal peptide synthetases (secondary metabolism). For these proteins to become functionally active, they need to be primed with the 4'-phosphopantetheine moiety of coenzyme A by a dedicated phosphopantetheine transferase (PPTase). Most organisms that employ more than one phosphopantetheinyl-dependent pathway also have more than one PPTase. Typically, one of these PPTases is optimized for the modification of carrier proteins of primary metabolism and rejects those of secondary metabolism (AcpS-type PPTases), whereas the other, Sfp-type PPTase, efficiently modifies carrier proteins involved in secondary metabolism. We present here a new type of PPTase, the carrier protein synthase of Pseudomonas aeruginosa, an organism that harbors merely one PPTase, namely PcpS. Gene deletion experiments clearly show that PcpS is essential for growth of P. aeruginosa, and biochemical data indicate its association with both fatty acid synthesis and siderophore metabolism. At first sight, PcpS is a PPTase of the monomeric Sfp-type and was consequently expected to have catalytic properties typical for this type of enzyme. However, in vitro characterization of PcpS with natural protein partners and non-cognate substrates revealed that its catalytic properties differ significantly from those of Sfp. Thus, the situation in P. aeruginosa is not simply the result of the loss of an AcpS-type PPTase. PcpS exhibits high catalytic efficiency with the carrier protein of fatty acid synthesis and shows a reduced although significant conversion rate of the carrier proteins of non-ribosomal peptide synthetases from their apo to holo form. This association with enzymes of primary and secondary metabolism indicates that PcpS belongs to a new sub-class of PPTases.