We have isolated and characterized cDNA clones encoding the H-protein of the glycine-cleavage system of pea (Pisum sativum) leaf mitochondria. The deduced primary structure revealed that the 131-amino-acid polypeptide is cytoplasmically synthesized with a 34-amino-acid mitochondrial targeting peptide. The lipoate-binding site was assigned to be lysine-63, as deduced from a sequence comparison with several lipoate-bearing proteins. The expression of the gene encoding H-protein was shown to occur specifically in the leaf tissue, with light exerting an additional effect by increasing the mRNA levels severalfold. Two polyadenylation sites were found in the mRNA, and a single-copy gene encoding the H-protein was detected in pea genome.