Cloning and expression of flavone synthase II from Gerbera hybrids.

In Gerbera hybrids, flavone synthesis is controlled by the locus Fns. The responsible enzyme, flavone synthase II, belongs to the NADPH-dependent cytochrome P450 monooxygenases. From two different chemogenetic defined Gerbera lines with the dominant (fns +.) or recessive (fns fns) alleles at the locus Fns, a cytochrome P450 fragment (CypDDd7a) was isolated using a differential display technique with upstream primers based on the conserved heme-binding region of cytochrome P450 proteins. The full-length cDNA (CYP93B2) which contained the open-reading frame and part of the CypDDd7a sequence was isolated via 5'-RACE and end-to-end PCR with gene specific primers. Northern blot analysis of total RNA of Gerbera hybrids indicated that the CYP93B2 gene was only transcribed in lines with the dominant allele fns + and that the transcript levels during flower development are in agreement with the measured enzyme activity of FNS II and flavone accumulation. Microsomes from yeast cells expressing CYP93B2 catalysed the direct formation of [14C]-flavones from the respective [14C]-flavanones. Thus, CYP93B2 was shown to encode flavone synthase II. This is the first report of the isolation and expression of a functional FNS II cDNA clone from any species. The comparison of amino acid sequences revealed that CYP93B2 had 54% identity with the sequence of CYP93B1, which has recently been reported as a (2S)-flavanone 2-hydroxylase of Glycyrrhiza echinata L.