Clock-associated genes in Arabidopsis: a family affair

The identification of components of the plant circadian clock has been adva nced recently with the success of oxo forward genetics approaches. The ZEIT LUPE and TOC1 loci were cloned and each was found to be part of two separat e, larger gene families with intriguing domain structures. The ZTL family o f proteins contains a subclass of the PAS domain coupled to an F box and ke lch motifs, suggesting that they play a role in a novel light-regulated ubi quitination mechanism. TOC1 shares similarity to the receiver domain of the well-known two-component phosphorelay signalling systems, combined with a strong similarity to a region of the CONSTANS transcription factor, which i s involved in controlling flowering time. When added to the repertoire of p reviously identified clock-associated genes, it is clear that both similari ties and differences with other circadian systems will emerge in the coming years.