title: Folate synthesis in plants: the last step of the p-aminobenzoate branch is catalyzed by a plastidial aminodeoxychorismate lyase.
abstract: In plants, the last step in the synthesis of p-aminobenzoate (PABA) moiety of folate remains to be elucidated. In Escherichia coli, this step is catalyzed by the PabC protein, a beta-lyase that converts 4-amino-4-deoxychorismate (ADC)--the reaction product of the PabA and PabB enzymes--to PABA and pyruvate. So far, the only known plant enzyme involved in PABA synthesis is ADC synthase, which has fused domains homologous to E. coli PabA and PabB and is located in plastids. ADC synthase has no lyase activity, implying that plants have a separate ADC lyase. No such lyase is known in any eukaryote. Genomic and phylogenetic approaches identified Arabidopsis and tomato cDNAs encoding PabC homologs with putative chloroplast-targeting peptides. These cDNAs were shown to encode functional enzymes by complementation of an E. coli pabC mutant, and by demonstrating that the partially purified recombinant proteins convert ADC to PABA. Plant ADC lyase is active as dimer and is not feedback inhibited by physiologic concentrations of PABA, its glucose ester, or folates. The full-length Arabidopsis ADC lyase polypeptide was translocated into isolated pea chloroplasts and, when fused to green fluorescent protein, directed the passenger protein to Arabidopsis chloroplasts in transient expression experiments. These data indicate that ADC lyase, like ADC synthase, is present in plastids. As shown previously for the ADC synthase transcript, the level of ADC lyase mRNA in the pericarp of tomato fruit falls sharply as ripening advances, suggesting that the expression of these two enzymes is coregulated.
authors.affiliation: Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA.
authors.forename: Gilles J C
authors.initials: GJ
authors.lastname: Basset
authors.forename: Stéphane
authors.initials: S
authors.lastname: Ravanel
authors.forename: Eoin P
authors.initials: EP
authors.lastname: Quinlivan
authors.forename: Ruth
authors.initials: R
authors.lastname: White
authors.forename: James J
authors.initials: JJ
authors.lastname: Giovannoni
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rébeillé
authors.forename: Brian P
authors.initials: BP
authors.lastname: Nichols
authors.forename: Kazuo
authors.initials: K
authors.lastname: Shinozaki
authors.forename: Motoaki
authors.initials: M
authors.lastname: Seki
authors.forename: Jesse F
authors.initials: JF
authors.lastname: Gregory
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Hanson
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology*; D002384:Catalysis; D018076:DNA, Complementary / Q000737:chemistry; D018744:DNA, Plant / Q000737:chemistry; D004926:Escherichia coli / Q000201:enzymology; D005492:Folic Acid / Q000096:biosynthesis*; D005638:Fruit / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D018551:Solanum lycopersicum / Q000201:enzymology*; D008969:Molecular Sequence Data; D009154:Mutation; D007652:Oxo-Acid-Lyases / Q000378:metabolism*; D010802:Phylogeny; D018087:Plastids / Q000201:enzymology*; D012333:RNA, Messenger / Q000378:metabolism; D018749:RNA, Plant / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D000637:Transaminases / Q000378:metabolism*
chemical_list: D018076:DNA, Complementary; D018744:DNA, Plant; D012333:RNA, Messenger; D018749:RNA, Plant; D011994:Recombinant Proteins; D005492:Folic Acid; C022309:PABA synthase; D000637:Transaminases; D007652:Oxo-Acid-Lyases; C066600:aminodeoxychorismate lyase