Rhythmic biological phenomena which occur at approximately 24 h intervals in the absence of environmental time cues are termed circadian rhythms. Two basic questions about circadian rhythms concern the mechanisms by which: 1) time information is used to regulate biological phenomena observed to be rhythmic, and 2) time is measured. To address the former, genes have been identified whose expression is regulated in a circadian fashion in Arabidopsis thaliana. Elements will be characterized that are required, in cis or in trans, for this circadian regulation using gene fusion, gel retardation and DNA protection strategies. The mechanism of time measurement will be addressed through the identification and characterization of mutations which disrupt biological timekeeping. The circadian rhythm in nitrate reductase will be explored to see if the protein expression and mRNA levels correspond. If they do correspond, nitrate reductase levels will be measured in an attempt to screen for circadian regulation mutations. Fusion genes, in which clock control is conferred on a reporter gene will be used as molecular tags on the biological clock in screens designed to identify mutations which affect clock function. The circadian rhythm in stomatal apertures will be used to select mutants in which circadian regulation is altered. Characterization of these mutations will identify elements either of the clock itself or of the regulatory system by which time control is exerted. *** //
