Isolation and characterization of two germacrene A synthase cDNA clones from chicory.
Chicory (Cichorium intybus) sesquiterpene lactones were recently shown to be derived from a common sesquiterpene intermediate, (+)-germacrene A. Germacrene A is of interest because of its key role in sesquiterpene lactone biosynthesis and because it is an enzyme-bound intermediate in the biosynthesis of a number of phytoalexins. Using polymerase chain reaction with degenerate primers, we have isolated two sesquiterpene synthases from chicory that exhibited 72% amino acid identity. Heterologous expression of the genes in Escherichia coli has shown that they both catalyze exclusively the formation of (+)-germacrene A, making this the first report, to our knowledge, on the isolation of (+)-germacrene A synthase (GAS)-encoding genes. Northern analysis demonstrated that both genes were expressed in all chicory tissues tested albeit at varying levels. Protein isolation and partial purification from chicory heads demonstrated the presence of two GAS proteins. On MonoQ, these proteins co-eluted with the two heterologously produced proteins. The K(m) value, pH optimum, and MonoQ elution volume of one of the proteins produced in E. coli were similar to the values reported for the GAS protein that was recently purified from chicory roots. Finally, the two deduced amino acid sequences were modeled, and the resulting protein models were compared with the crystal structure of tobacco (Nicotiana tabacum) 5-epi-aristolochene synthase, which forms germacrene A as an enzyme-bound intermediate en route to 5-epi-aristolochene. The possible involvement of a number of amino acids in sesquiterpene synthase product specificity is discussed.
Bouwmeester
Harro J
HJ
Plant Research International, Business Unit Cell Cybernetics, P.O. Box 16, 6700 AA Wageningen, The Netherlands. h.j.bouwmeester@plant.wagur.nl
Kodde
Jan
J
Verstappen
Francel W A
FW
Altug
Iris G
IG
de Kraker
Jan-Willem
JW
Wallaart
T Eelco
TE
D019883:Alkyl and Aryl Transferases / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D000595:Amino Acid Sequence; D018651:Cichorium intybus / Q000201:enzymology* / Q000235:genetics; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification*; D004926:Escherichia coli / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D007527:Isoenzymes / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D008954:Models, Biological; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D010940:Plant Proteins; D011487:Protein Conformation; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D012717:Sesquiterpenes / Q000138:chemical synthesis; D014026:Nicotiana / Q000201:enzymology / Q000235:genetics
D018076:DNA, Complementary; D007527:Isoenzymes; D010940:Plant Proteins; D012717:Sesquiterpenes; D019883:Alkyl and Aryl Transferases; C078217:5-epi-aristolochene synthase; C462022:germacrene A synthase, Cichorium intybus