The Arabidopsis CAB2 promoter was used to define the terminal genomic targets that are subject to regulation by the circadian clock. An in vivo cab::luciferase bioluminescent marker was used to enable the assaying of the expression of chimeric constructs with unprecedented sensitivity and time resolution in living seedlings. Dissection of -322 to +1 of the CAB2 promoter has revealed several interesting features: it was demonstrated that the 323 bp fragment contains at least one strong general positive element. The positive element contains an ACGT core sequence specifically bound by a protein activity, termed CUF-1, and contributes to high level expression but is not required for phytochrome- or circadian-regulation. Moreover, a 78 bp domain was defined that confers both circadian- and phytochrome-regulation upon heterologous promoters. Conserved GATA sequences within the 78 bp regulatory domain are specifically bound by a protein factor designated CGF-1. The binding specificity of CGF-1 appears to be related to the GT-family of trihelix DNA-binding proteins. The role of these DNA-protein interactions is discussed in terms of clock- and phytochrome regulation, and their relevance as targets for pathways defined by photomorphogenic mutants.