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February Volume 18 Number 2 pp 208 - 212
Supplement to: Identification and expression of isoflavone synthase, the key enzyme for biosynthesis of isoflavones in legumes

Relationship of IFS1 to other cytochromes P450

The cDNA identified by this functional screening as encoding a protein with isoflavone synthase activity was fully sequenced and shown to be closely related to cyp93c1, a soybean cDNA with no reported enzyme activity (accession number AF022462; ref. 1). Cytochromes P450 are named on the basis of structural similarity, not functional similarity (, although proteins with a high degree of structural similarity may be expected to have some functional relatedness. Cytochrome P450 proteins that are greater then 40% identical are placed in the same family, while proteins that are greater then 55% identical are placed in the same subfamily. The cyp93 family has thirteen members, divided into four subfamilies, for which full-length sequences are deposited with the cytochrome P450 nomenclature committee The cyp93a subfamily has four members, only one of which has been functionally identified. One of these genes, isolated from soybean, encodes dihydroxypterocarpan 6a-hydroxylase, an enzyme in the pathway that converts daidzein to glyceollins2. The cyp93b subfamily has five members, one of which has been functionally identified. Cyp93b1, isolated from licorice, encodes (2S)-flavanone 2-hydroxylase. This enzyme converts the hydroxyflavanones naringenin and liquiritigenin, the same substrates used by IFS, to flavones3. The cyp93c subfamily has three members other than cyp93c1, none of which has been functionally identified. The cyp93d subfamily has only one member, which has been identified through genomic sequencing of Arabidopsis. Neither a cDNA nor an enzymatic activity has been reported for this genomic sequence. The identification of IFS as being a member of the cyp93 family is consistent with the roles that the other characterized members of this family play in the phenylpropanoid pathway. Though IFS1 and cyp93b1 act on the same substrates, they have only 48% amino acid identity.

  1. Siminszky, B., Corbin, F.T., Ward, E.R., Fleischmann, T.J. & Dewey, R.E. Expression of a soybean cytochrome P450 monooxygenase cDNA in yeast and tobacco enhances the metabolism of phenylurea herbicides. . Proc. Natl. Acad. Sci. U.S.A., 96 1750-1755 (1999).
  2. Schopfer, C.R., Koch, G., Lottspeich, F. & Ebel, J. Molecular characterization and functional expression of dihydroxypterocarpan 6a-hydroxylase, an enzyme specific for pterocarpanoid phytoalexin biosynthesis in soybean (Glycine max L.). FEBS Lett., 432, 182-186 (1998).
  3. Akashi, T., Aoki, T. & Ayabe, S. Identification of a cytochrome P450 cDNA encoding (2S)-flavanone 2-hydroxylase of licorice (Glycyrrhiza echinata L.; Fabaceae) which represents licodione synthase and flavone synthase II. . FEBS Lett., 431, 287-290 (1998).




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