Duplication and functional divergence in the chalcone synthase gene family of Asteraceae: Evolution with substrate change and catalytic simplification

Plant-specific polyketide synthase genes constitute a gene superfamily, including universal chalcone synthase [CHS; malonyl CoA:4-coumaroyl-CoA malonyltransferase (cyclizing) (EC 2.3.1.74)] genes, sporadically distributed stilbene synthase (SS) genes, and atypical, as-yet-uncharacterized CHS-like genes, We have recently isolated from Gerbera hybrida (Asteraceae) an unusual CHS-like gene, GCHS2, which codes for an enzyme with structural and enzymatic properties as well as ontogenetic distribution distinct from both CHS and SS, Here, we show that the GCHS2-like function is encoded in the Gerbera genome by a family of at least three transcriptionally active genes, Conservation within the GCHS2 family was exploited with selective PCR to study the occurrence of GCHS2-like genes in other Asteraceae. Parsimony analysis of the amplified sequences together with CHS-like genes isolated from other taxa of angiosperm sub-class Asteridae suggests that GCHS2 has evolved from CHS via a gene duplication event that occurred before the diversification of the Asteraceae, Enzyme activity analysis of proteins produced in vitro indicates that the GCHS2 reaction is a non-SS variant of the CHS reaction, with both different substrate specificity (to benzoyl-CoA) and a truncated catalytic profile, Together with the recent results of Durbin ed al, [Durbin, M. L., Learn, G. H., Jr., Huttley, G. A. & Clegg, M. T. (1995) Proc. Natl. Acad. Sci. USA 92, 3338-3342], our study confirms a gene duplication-based model that explains how various related functions have arisen from CHS during plant evolution.