New light is shining on the melatonin rhythm enzyme: the first postcloning view.

One of the most interesting molecules in circadian biology is serotonin N-acetyltransferase (arylalkyfamine N-acetyltransferase, AANAT), the enzyme that controls the daily rhythm in pineal melatonin production and blood melatonin. The recent cloning of AANAT cDNA has led to the characterization of the human gene; the realization that AANAT represents a unique gene family; the discovery of circadian rhythms in AANAT mRNA; the determination of the basis of transsynaptic and cellular regulation of expression of the AANAT gene; a new understanding of the relationship of AANAT mRNA and activity; and the surprising finding of strong expression of the AANAT gene in the retina and significant levels in select brain regions, the pituitary gland, and testes. The cloning of AANAT cDNA has not only made it possible to answer longstanding questions in circadian biology, but has also raised stimulating new issues.