Abstract
Melatonin biosynthesis involves the N-acetylation of arylalkylamines such as serotonin, which is catalysed by serotonin N-acetyltransferase (SNAT), the penultimate enzyme of melatonin biosynthesis in both animals and plants. Here, we report the functional characterization of a putative N-acetyltransferase gene in the chloroplast genome of the alga laver (Pyropia yezoensis, formerly known as Porphyra yezoensis) with homology to the rice SNAT gene. To confirm that the putative Pyropia yezoensis SNAT (PySNAT) gene encodes an SNAT, we cloned the full-length chloroplastidic PySNAT gene by PCR and purified the recombinant PySNAT protein from Escherichia coli. PySNAT was 174 aa and had 50% amino acid identity with cyanobacteria SNAT. Purified recombinant PySNAT showed a peak activity at 55 °C with a K m of 467 µM and V max of 28 nmol min-1 mg(-1) of protein. Unlike other plant SNATs, PySNAT localized to the cytoplasm due to a lack of N-terminal chloroplast transit peptides. Melatonin was present at 0.16ng g(-1) of fresh mass but increased during heat stress. Phylogenetic analysis of the sequence suggested that PySNAT has evolved from the cyanobacteria SNAT gene via endosymbiotic gene transfer. Additionally, the chloroplast transit peptides of plant SNATs were acquired 1500 million years ago, concurrent with the appearance of green algae.
Highlights
Pyropia yezoensis, a marine alga laver known previously as Porphyra yezoensis (Sutherland et al, 2011), is used commercially in foods such as sushi
Melatonin biosynthesis involves the N-acetylation of arylalkylamines such as serotonin, which is catalysed by serotonin N-acetyltransferase (SNAT), the penultimate enzyme of melatonin biosynthesis in both animals and plants
Characterization of the SNAT gene in the Pyropia yezoensis chloroplast genome The rice SNAT gene exists as a single copy in the rice genome and is highly conserved in plant lineages, including cyanobacteria (Byeon et al, 2013)
Summary
A marine alga laver known previously as Porphyra yezoensis (Sutherland et al, 2011), is used commercially in foods such as sushi. The products of laver are valued at around US$2 billion within East Asia (Sahoo et al, 2002). Due to its economic importance and nutritional properties, Pyropia cultivation is expanding to other countries (Noda, 1993). The thalli are in the gametophytic phase and consist of one or two thick cells containing one or two chloroplasts (Sahoo et al, 2002; Sutherland et al, 2011). Pyropia is known to be higher in vitamin C than oranges (Noda, 1993)
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