Introducing Aromatic Amino Acid Hydroxylases from Plants

merization domain (reviewed in Fitzpatrick, 2003), these domains were not present in plant AAHs. In addition, the plant AAHs contained a nonconserved sequence in the N terminus that was predicted to be a plastid-targeting peptide. Functional complementation of an Escherichia coli Tyr auxotroph demonstrated that the plant AAHs had Phe hydroxylase activity. Furthermore, analytical size exclusion chromatography showed that, in contrast with animal AAHs, which occur as homotetramers (reviewed in Fitzpatrick, 2003), plant AAHs function as monomers. Treatment with various chelators demonstrated that the activity of plant AAHs, like that of animal AAHs, depends on iron. Next, the authors tested a series of pterins and folates for their ability to act as a cofactor for pine and moss AAHs and identified a folate, 10-formyltetrahydrofolate, as the physiological cofactor of these enzymes. As suggested by the presence of a putative plastid-targeting peptide, targeting assays using full-length green fluorescent protein (GFP)‐tagged pine and moss AAHs in Arabidopsis thaliana mesophyll cells (see figure), and import assays using purified pea (Pisum sativum) chloroplasts