MOLECULAR-CLONING AND CHARACTERIZATION OF A CDNA-ENCODING THE RETINALARYLALKYLAMINE N-ACETYLTRANSFERASE OF THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Melatonin synthesis in the retina as well as in the pineal gland exhib its daily variations with higher levels during the dark phase of light -dark cycles. To analyze the molecular mechanism of melatonin synthesi s in the retina, we have cloned, sequenced and characterized a putativ e cDNA for arylalkylamine N-acetyltransferase (AANAT; EC 2.3.1.87), a rate-limiting enzyme in melatonin production, from the retina of the r ainbow trout (Oncorhynchus mykiss). The trout AANAT cDNA (1,585 bp) co ntains an open reading frame encoding 240 amino acid protein (predicte d molecular weight, 27,420) that is 51-65% identical to avian and mamm alian AANAT. The trout retinal AANAT protein contains motifs A and B t hat are conserved among the N-acetyltransferase superfamily and eight potential phosphorylation sites. Southern blot analysis demonstrated t hat the protein is expressed by a single copy gene. A single AANAT tra nscript (1.6 kb) was detected in the retina but not in the liver by No rthern blot analysis. The levels of AANAT mRNA in the retina exhibited day-night changes with 3.3-fold increase at night. These results indi cate that in the rainbow trout retina, the activity of AANAT and thus melatonin synthesis are regulated at least in part at the transcriptio nal level.