title: Folate synthesis in plants: the first step of the pterin branch is mediated by a unique bimodular GTP cyclohydrolase I.
abstract: GTP cyclohydrolase I (GCHI) mediates the first and committing step of the pterin branch of the folate-synthesis pathway. In microorganisms and mammals, GCHI is a homodecamer of approximately 26-kDa subunits. Genomic approaches identified tomato and Arabidopsis cDNAs specifying approximately 50-kDa proteins containing two GCHI-like domains in tandem and indicated that such bimodular proteins occur in other plants. Neither domain of these proteins has a full set of the residues involved in substrate binding and catalysis in other GCHIs. The tomato and Arabidopsis cDNAs nevertheless encode functional enzymes, as shown by complementation of a yeast fol2 mutant and by assaying GCHI activity in extracts of complemented yeast cells. Neither domain expressed separately had GCHI activity. Recombinant tomato GCHI formed dihydroneopterin triphosphate as reaction product, as do other GCHIs, but unlike these enzymes it did not show cooperative behavior and was inhibited by its substrate. Denaturing gel electrophoresis verified that the bimodular GCHI polypeptide is not cleaved in vivo into its component domains, and size-exclusion chromatography indicated that the active enzyme is a dimer. The deduced tomato and Arabidopsis GCHI polypeptides lack overt targeting sequences and thus are presumably cytosolic, in contrast to other plant folate-synthesis enzymes, which are mitochondrial proteins with typical signal peptides. GCHI mRNA and protein are strongly in expressed unripe tomato fruits, implying that fruit folate is made in situ rather than imported. As ripening advances, GCHI expression declines sharply, and folate content drops, suggesting that folate synthesis fails to keep pace with turnover.
authors.affiliation: Horticultural Sciences Department and Food Science and Human Nutrition Department, University of Florida, Gainesville, FL 32611, USA.
authors.forename: Gilles
authors.initials: G
authors.lastname: Basset
authors.forename: Eoin P
authors.initials: EP
authors.lastname: Quinlivan
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ziemak
authors.forename: Rocio
authors.initials: R
authors.lastname: Diaz De La Garza
authors.forename: Markus
authors.initials: M
authors.lastname: Fischer
authors.forename: Susi
authors.initials: S
authors.lastname: Schiffmann
authors.forename: Adelbert
authors.initials: A
authors.lastname: Bacher
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 / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D001483:Base Sequence; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D019281:Dimerization; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005492:Folic Acid / Q000096:biosynthesis*; D006136:GTP Cyclohydrolase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D006160:Guanosine Triphosphate / Q000494:pharmacology; D018551:Solanum lycopersicum / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D011622:Pterins / Q000737:chemistry / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D018749:RNA, Plant / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity
chemical_list: D018076:DNA, Complementary; D018744:DNA, Plant; D004791:Enzyme Inhibitors; D011622:Pterins; D012333:RNA, Messenger; D018749:RNA, Plant; D006160:Guanosine Triphosphate; D005492:Folic Acid; D006136:GTP Cyclohydrolase