Functional Characterization of Serotonin -Acetyltransferase in Archaeon

Serotonin -acetyltransferase is the penultimate enzyme in the melatonin biosynthetic pathway that catalyzes serotonin into -acetylserotonin. Many genes have been cloned and characterized from organisms ranging from bacteria to plants and mammals. However, to date, no gene has been identified from Archaea. In this study, three archaeal candidate genes were synthesized and expressed in , and SNAT enzyme activity was measured using their purified recombinant proteins. Two candidate genes, from Methanoregulaceae (Archaea) and , showed no SNAT enzyme activity, whereas a candidate gene from previously named exhibited SNAT enzyme activity. The substrate affinity and the maximum reaction rate of TvSNAT toward serotonin were 621 μM and 416 pmol/min/mg protein, respectively. The highest amine substrate was tyramine, followed by tryptamine, serotonin, and 5-methoxytryptamine, which were similar to those of plant SNAT enzymes. Homologs of were found in many Archaea families. Ectopic overexpression of in rice resulted in increased melatonin content, antioxidant activity, and seed size in conjunction with the enhanced expression of seed size-related gene. This study is the first to report the discovery of gene in Archaea. Future research avenues include the cloning of T orthologs in different phyla, and identification of their regulation and functions related to melatonin biosynthesis in living organisms.