Identification of genome targets of the DET1 complex in higher plants

Light provides a major source of information from the environment during plant growth and development. Light-regulated gene expression is partly controlled by the phytochrome photoreceptors, which once activated, are imported into the nucleus where they bind and activate transcription factors such as PIF3. Coupled with this, the degradation of positively acting intermediates such as the transcription factor HY5 by COP1 and other ubiquitin ligases acts to repress photomorphogenesis in darkness. Another negative regulator of photomorphogenesis is DET1 (De-Etiolated 1), which forms part of a nuclear-localised complex with the plant homolog of UV- Damaged DNA Binding protein 1 (DDB1) and the E2 ubiquitin-ligase activating protein COP10 (Constitutive Photomorphogenic 10). Previously it was found that DET 1 binds chromatin via a direct interaction with the core histone H2B, suggesting that DET1 may repress light activated genes by interacting directly with their promoters. Here it is shown that DET1 forms part of a CUL4-based ubiquitin ligase complex and is localised to discrete foci throughout the nucleus. Chromatin Immunoprecipitation experiments show that in the dark DET 1 binds to the promoters of the light induced CAB2 and HEMA1 genes, and that this binding is abolished in the light, coincidental with the activation of these genes. DET 1 was also detected at the promoter of the light-repressed POR-A gene when it is in its repressed state as well as the promoter of the developmental^ regulated FT gene, again when this gene is repressed. Based on these data it is proposed that the DET1 complex binds the promoters of light- regulated genes in the dark and directly represses their transcription, either by chromatin remodelling and/or ubiquitin-mediated regulation of the transcriptional apparatus. Light causes the dissociation of the DET 1 complex from these promoters, allowing activation of these genes. DET1 may also play a more general role in genome maintenance during plant development.