Light is an important environmental cue to plants, and much of their physiology is influenced by light. The light signals that drive these responses are perceived by photoreceptors including the red/far-red responsive phytochromes (phyA-E). In addition to direct effects, light also exerts its influence by modifying the rhythms generated by the circadian clock. In Arabidopsis thaliana, the molecular makeup of the interface between the central clock and its input/output pathways is not fully defined, but a major point of control is likely to be protein turnover mediated by the ubiquitin/26S proteasome system. To identify additional constituents of this interface, stable double-stranded RNA interference (RNAi) was used to reduce mRNA levels of rhythmically expressed candidate genes encoding putative components of E3 ubiquitin ligases (i.e., F box and RING finger proteins), followed by screening of the transgenic plants for circadian and light signaling defects. RNAi lines with diminished expression of the novel gene ATTENUATED FAR-RED RESPONSE (AFR) display phenotypes consistent with impaired phyA-mediated light signaling. Furthermore, AFR is a true SCF E3 ubiquitin ligase component. SCF(AFR) is expected to mediate the turnover of a repressor of phyA signaling, possibly to prepare the plant to receive light signals at dawn.