| Summary: | Serotonin <i>N</i>-acetyltransferase is the penultimate enzyme in the melatonin biosynthetic pathway that catalyzes serotonin into <i>N</i>-acetylserotonin. Many <i>SNAT</i> genes have been cloned and characterized from organisms ranging from bacteria to plants and mammals. However, to date, no <i>SNAT</i> gene has been identified from Archaea. In this study, three archaeal <i>SNAT</i> candidate genes were synthesized and expressed in <i>Escherichia coli</i>, and SNAT enzyme activity was measured using their purified recombinant proteins. Two <i>SNAT</i> candidate genes, from Methanoregulaceae (Archaea) and <i>Pyrococcus furiosus</i>, showed no SNAT enzyme activity, whereas a <i>SNAT</i> candidate gene from <i>Thermoplasma volcanium</i> previously named <i>TvArd1</i> 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 <i>TvSNAT</i> were found in many Archaea families. Ectopic overexpression of <i>TvSNAT</i> 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 <i>SNAT</i> gene in Archaea. Future research avenues include the cloning of <i>TvSNA</i>T orthologs in different phyla, and identification of their regulation and functions related to melatonin biosynthesis in living organisms.
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