| Summary: | Serotonin <i>N</i>-acetyltransferase (SNAT) functions as the penultimate or final enzyme in melatonin biosynthesis, depending on the substrate. The <i>Escherichia coli</i> orthologue of archaeal SNAT from <i>Thermoplasma volcanium</i> was identified as RimI (EcRimI), with 42% amino acid similarity to archaeal SNAT. EcRimI has been reported to be an <i>N</i>-acetyltransferase enzyme. Here, we investigated whether EcRimI also exhibits SNAT enzyme activity. To achieve this goal, we purified recombinant EcRimI and examined its SNAT enzyme kinetics. As expected, EcRimI showed SNAT activity toward various amine substrates including serotonin and 5-methoxytryptamine, with <i>K</i><sub>m</sub> and <i>V</i><sub>max</sub> values of 531 μM and 528 pmol/min/mg protein toward serotonin and 201 μM and 587 pmol/min/mg protein toward 5-methoxytryptamine, respectively. In contrast to the <i>rimI</i> mutant <i>E. coli</i> strain that showed no growth defect, the <i>EcRimI</i> overexpression strain exhibited a 2-fold higher growth rate than the control strain after 24 h incubation in nutrient-rich medium. The <i>EcRimI</i> overexpression strain produced more melatonin than the control strain in the presence of 5-methoxytryptamine. The enhanced growth effect of <i>EcRimI</i> overexpression was also observed under cadmium stress. The higher growth rate associated with <i>EcRimI</i> expression was attributed to increased protein <i>N</i>-acetyltransferase activity, increased synthesis of melatonin, or the combined effects of both.
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