CINNAMOYL COA REDUCTASE, THE FIRST COMMITTED ENZYME OF THE LIGNIN BRANCH BIOSYNTHETIC-PATHWAY - CLONING, EXPRESSION AND PHYLOGENETIC-RELATIONSHIPS

Cinnamoyl CoA:NADP oxidoreductase (CCR, EC 1.2.1.44) catalyzes the con version of cinnamoyl CoA esters to their corresponding cinnamaldehydes , i.e. the first specific step in the synthesis of the lignin monomers . The cloning of a cDNA encoding CCR in Eucalyptus gunnii (EUCCR) is r eported here. The identity of the EUCCR cDNA was demonstrated by compa rison with peptide sequence data from purified CCR and functional expr ession of the recombinant enzyme in Escherichia coli. Sequence analysi s revealed remarkable homologies with dihydroflavonol-4-reductase (DFR ), the first enzyme of the anthocyanin biosynthetic pathway. Moreover, significant similarities were found with mammalian 3 beta-hydroxyster oid dehydrogenase and bacterial UDP-galactose-4-epimerase, suggesting that CCR shared a common ancestor with these enzymes and can therefore be considered as a new member of the mammalian BP-hydroxysteroid dehy drogenase/plant dihydroflavonol reductase superfamily. In Eucalyptus g unnii, CCR is encoded by one gene containing four introns whose positi ons are similar to those of introns I, II, III and V in DFR genes from dicots. In agreement with the involvement of CCR in lignification, th e CCR transcript was shown to be expressed in lignified organs, i.e. r oot and stem tissues, and was localized mainly in young differentiatin g xylem. On the other hand, its abundance in Eucalyptus leaves suggest s that monolignols may be precursors of end products other than lignin s. This first characterization of a gene corresponding to CCR opens ne w possibilities to genetically engineer plants with lower lignin conte nt. This is particularly important for woody plants such as Eucalyptus which are used for pulp making.