title: Two O-methyltransferases involved in the biosynthesis of methoxypyrazines: grape-derived aroma compounds important to wine flavour.
abstract: Methoxypyrazines (MPs) are volatile, grape-derived aroma compounds that contribute to the distinct herbaceous characters of some wines. Although the full pathway leading to MP production has not been elucidated, there is strong evidence that the final step involves the methylation of non-volatile hydroxypyrazine (HP) precursors. Two cDNA encoding O-methyltransferases (OMTs) that have homology to an enzyme previously purified and shown to catalyse the methylation of HPs were isolated from Cabernet Sauvignon. Recombinant protein from the cDNAs (VvOMT1 and VvOMT2) was produced in E. coli and activity assays demonstrated that both encode OMTs able to methylate HPs to produce MPs, however both showed greatest activity against the flavonol quercetin. VvOMT1 has higher catalytic activity against isobutyl hydroxypyrazine compared to isopropyl hydroxypyrazine, whereas the converse is true for VvOMT2. The timing of the expression of VvOMT1 in the skin and the flesh of developing Cabernet Sauvignon grape berries was associated with the period of MP accumulation in these tissues, while VvOMT2 expression was greatest in roots, which were found to contain high levels of MPs. The MP composition of these tissues also reflects the relative levels of expression of these genes and their substrate preference. The identification of genes responsible for MP production in grapevine will help in understanding the effect of different viticultural and environmental factors on MP accumulation.
authors.affiliation: Flinders University of South Australia, Adelaide, SA, Australia.
authors.forename: Jake D
authors.initials: JD
authors.lastname: Dunlevy
authors.forename: Kathleen L
authors.initials: KL
authors.lastname: Soole
authors.forename: Michael V
authors.initials: MV
authors.lastname: Perkins
authors.forename: Eric G
authors.initials: EG
authors.lastname: Dennis
authors.forename: Robert A
authors.initials: RA
authors.lastname: Keyzers
authors.forename: Curtis M
authors.initials: CM
authors.lastname: Kalua
authors.forename: Paul K
authors.initials: PK
authors.lastname: Boss
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D005421:Flavoring Agents / Q000378:metabolism; D017343:Genes, Plant; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D011719:Pyrazines / Q000737:chemistry / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D027843:Vitis / Q000235:genetics / Q000378:metabolism*; D014920:Wine / Q000032:analysis
chemical_list: D017931:DNA Primers; D018076:DNA, Complementary; D018744:DNA, Plant; D005421:Flavoring Agents; D010940:Plant Proteins; D011719:Pyrazines; D011994:Recombinant Proteins; D008780:Methyltransferases