Aminolevulinic acid dehydratase in pea (Pisum sativum L.): Identification of an unusual metal-binding domain in the plant enzyme

Aminolevulinic acid dehydratase (ALA dehydratase) catalyzes the second step of tetrapyrrole synthesis leading to the formation of heme and chlorophyll in higher plant cells. Antibodies elicited against spinach leaf ALA dehydratase were used to immunoscreen λgt11 cDNA libraries constructed from etiolated pea (Pisum sativum L.) leaf poly(A)⁺ RNAs. A set of overlapping cDNAs was characterized that encode the pea enzyme. The predicted amino acid sequence of the pea ALA dehydratase is similar to those reported for other eukaryotic and prokaryotic ALA dehydratases. The pea enzyme has an active site domain centered on lysine that is highly conserved in comparison to other known ALA dehydratases. Consistent with the previously reported requirement of Mg²⁺ for catalytic activity by plant ALA dehydratases, the pea enzyme lacks the characteristic Zn²⁺-binding domain present in other eukaryotic ALA dehydratases, but contains a distinctive metal ligand-binding domain based upon aspartate. Northern blot analyses demonstrated that ALA dehydratase mRNA is present in leaves, stems, and to a lesser extent in roots. Steady state levels of mRNA encoding ALA dehydratase exhibit little or no change during light-induced greening.