Department of Biochemistry, School of Medical Sciences, University of Bristol, U.K.
A full-length protochlorophyllide reductase clone (pWR5) has been isolated from a dark-grown wheat (Triticum aestivum) cDNA library generated in the phage vector lambda gt10. Comparison of the sequence of pWR5 with published sequences indicates a high degree of conservation of the structure of the mature protein amongst species but with the structure of the transit peptide less highly conserved. Within the cereals, the structure of the complete preprotein shows a remarkable degree of sequence homology (98% between barley and wheat). In vitro expression of pWR5 generates a preprotein of the expected molecular mass, approx. 41 kDa. Isolated pea chloroplasts can import, process and locate the mature reductase to the thylakoid membranes. From analysis of the CNBr-cleavage fragments of the N-[3H]phenylmaleimide-treated enzyme, the substrate-protected cysteine group in the enzyme is tentatively identified as Cys-296.