Functional Characterization of Arylalkylamine N -Acetyltransferase, a Pivotal Gene in Antioxidant Melatonin Biosynthesis from Chlamydomonas reinhardtii .

Arylalkylamine N -acetyltransferase (AANAT) is a pivotal enzyme in melatonin biosynthesis that catalyzes the conversion of serotonin to N -acetylserotonin. Homologs of animal AANAT genes are present in animals, but not in plants. An AANAT homolog was found in Chlamydomonas reinhardtii , but not other green algae. The characteristics of C. reinhardtii   AANAT ( CrAANAT ) are unclear. Here, full-length CrAANAT was chemically synthesized and expressed in Escherichia coli . Recombinant CrAANAT exhibited AANAT activity with a K m of 247 μM and V max of 325 pmol/min/mg protein with serotonin as the substrate. CrAANAT was localized to the cytoplasm in tobacco leaf cells. Transgenic rice plants overexpressing CrAANAT ( CrAANAT -OE) exhibited increased melatonin production. CrAANAT -OE plants showed a longer seed length and larger second leaf angle than wild-type plants, indicative of the involvement of brassinosteroids (BRs). As expected, BR biosynthesis- and signaling-related genes such as D2 , DWARF4 , DWARF11 , and BZR1 were upregulated in CrAANAT -OE plants. Therefore, an increased endogenous melatonin level by ectopic overexpression of CrAANAT seems to be closely associated with BR biosynthesis, thereby influencing seed size.