Catalyzes the N-acetylation of serotonin into N-acetylserotonin, the penultimate step in the synthesis of melatonin. Catalyzes in vitro the N-acetylation of tryptamine to produce N-acetyltryptamine, 5-methoxytryptamine to produce melatonin and tyramine to produce N-acetyltyramine (By similarity). Acetyltransferase required for geminivirus infection and systemic spread (By similarity). 2-arylethylamine + acetyl-CoA = CoA + H(+) + N-acetyl-2-arylethylamine Aromatic compound metabolism; melatonin biosynthesis; melatonin from serotonin: step 1/2.