The molecular ethology of the period gene in Drosophila

The period (per) gene in Drosophila has made a major contribution to the emerging field of molecular ethology. Mutations at this locus after a number of temporally programmed phenotypes, such as the 24-h circadian locomotor activity cycle and the 60-s male love-song cycle. Molecular analysis of per has revealed that it encodes a conceptual protein of approximately 1200 amino acids. A striking feature of the per protein is a series of alternating threonine-glycine residues, which, when removed by in vitro mutagenesis, alter song periodicity but not circadian periodicity. Evolutionary implications of this, and of species differences among per genes, are discussed. The threonine-glycine region is similar to a serine-glycine repeat found in mammalian proteoglycans. Based on this observation and other experiments, a model has been proposed, which suggests that the per protein may determine periodicity by modulating intercellular communication. The per protein is localized in the eye, brain, and other nonneural tissues, and immunochemical staining appears to cycle in intensity in the visual system and some brain cells. Perhaps some of these per+staining cells represent the anatomical concomitant of "the clock."