Molecular cloning and functional analysis of serotonin N‐acetyltransferase from the cyanobacterium Synechocystis sp. PCC 6803

Serotonin N‐acetyltransferase (SNAT) catalyzes conversion of serotonin into N‐acetylserotonin, which is a direct precursor for melatonin biosynthesis in all organisms. Molecular cloning of plant SNAT from rice led to a screening for SNAT homolog genes in other species. We identified a cyanobacterium SNAT‐like gene (cSNAT) that showed 56% amino acid homology with the rice SNAT. To confirm whether cSNAT encoded SNAT enzyme activity, we expressed cSNAT DNA in Escherichia coli and purified the cSNAT protein as a C‐terminal His‐tagged form. The purified cSNAT protein exhibited SNAT enzyme activities, transferring the acetyl group into either serotonin or tryptamine substrates. The optimum temperature was 55°C, but it was still highly active at 70°C, suggesting that cSNAT is a thermotolerant enzyme. The K_m and V_max were 823 μm and 1.6 nmol/min/mg protein, respectively. The cSNAT gene is highly conserved in all cyanobacterial taxa and seems to be an origin of SNAT in higher plants. The thermotolerance of cSNAT suggests that melatonin plays a role in the response to high‐temperature stress. Further analysis of this role of melatonin in higher plants is needed.