The primary structure of the NADPH-protochlorophyllide oxidoreductase of barley has been deduced from the nucleotide sequence of a cloned full-length cDNA. This cDNA hybridizes to a 1.7 kb RNA whose steady-state level in dark-grown seedlings is drastically reduced upon illumination. The predicted amino acid sequence (388 residues in length) includes a transit peptide of 74 amino acids whose end point has been delimited by sequencing the N-terminus of the mature protein. Expression of the cDNA inEscherichia coli leads to the synthesis of an enzymatically active precursor of the NADPH-protochlorophyllide oxidoreductase. Activity of this protein in bacterial lysates is completely dependent on the presence of NADPH and protochlorophyllide and requires light.