Abstract
A new regulatory element necessary for the correct temporal expression of the period (per) gene was identified by monitoring real‐time per expression in living individual flies carrying two different period–luciferase transgenes. luciferase RNA driven from only the per promoter was not sufficient to replicate the normal pattern of per RNA cycling; however, a per–luc fusion RNA driven from a transgene containing additional per sequences cycled identically to endogenous per. The results indicate the existence of at least two circadian‐regulated elements—one within the promoter and one within the transcribed portion of the per gene. Phase and amplitude analysis of both per–luc transgenes revealed that normal per expression requires the regulation of these elements at distinct phases and suggests a mechanism by which biological clocks sustain high‐amplitude feedback oscillations.