Folate synthesis in plants: The p-aminobenzoate branch is initiated by a bifunctional PabA-PabB protein that is targeted to plastids

• Published in: Proceedings of the National Academy of Sciences - PNAS 2004-02, Vol.101 (6), p.1496-1501
• Main Authors: Basset, G.J.C, Quinlivan, E.P, Ravanel, S, Rebeille, F, Nichols, B.P, Shinozaki, K, Seki, M, Adams-Phillips, L.C, Giovannoni, J.J, Gregory, J.F. III
• Format: Article
• Language: English
• Subjects: 4-Aminobenzoic Acid - metabolism; Arabidopsis thaliana; Bacteria; Base Sequence; Biochemistry; Biological Sciences; Botany; Cellular Biology; Complementary DNA; DNA Primers; Enzymes; Fluorescence; Folic Acid - biosynthesis; Genetic Complementation Test; Leaves; Life Sciences; Lycopersicon esculentum; Messenger RNA; Molecular Sequence Data; p-aminobenzoate; PabA protein; PabB protein; PabC protein; Plant Proteins - metabolism; Plants; Plants - enzymology; Plants - metabolism; Plastids; Plastids - metabolism; Proteins; Reverse Transcriptase Polymerase Chain Reaction; Ripening; Subcellular Fractions - enzymology; Yeasts
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0308331100

It is not known how plants synthesize the p-aminobenzoate (PABA) moiety of folates. In Escherichia coli, PABA is made from chorismate in two steps. First, the PabA and PabB proteins interact to catalyze transfer of the amide nitrogen of glutamine to chorismate, forming 4-amino-4-deoxychorismate (ADC). The PabC protein then mediates elimination of pyruvate and aromatization to give PABA. Fungi, actinomycetes, and Plasmodium spp. also synthesize PABA but have proteins comprising fused domains homologous to PabA and PabB. These bipartite proteins are commonly called "PABA synthases," although it is unclear whether they produce PABA or ADC. Genomic approaches identified Arabidopsis and tomato cDNAs encoding bipartite proteins containing fused PabA and PabB domains, plus a putative chloroplast targeting peptide. These cDNAs encode functional enzymes, as demonstrated by complementation of an E. coli pabA pabB double mutant and a yeast PABA-synthase deletant. The partially purified recombinant Arabidopsis protein did not produce PABA unless the E. coli PabC enzyme was added, indicating that it forms ADC, not PABA. The enzyme behaved as a monomer in size-exclusion chromatography and was not inhibited by physiological concentrations of PABA, its glucose ester, or folates. When the putative targeting peptide was fused to GFP and expressed in protoplasts, the fusion protein appeared only in chloroplasts, indicating that PABA synthesis is plastidial. In the pericarp of tomato fruit, the PabA-PabB mRNA level fell drastically as ripening advanced, but there was no fall in total PABA content, which stayed between 0.7 and 2.3 nmol· g-1fresh weight.