Ubiquitin is a small polypeptide and ubiquitination is the post-translational modification by ubiquitin protein, resulting in degradation of target proteins by the 26S proteasome complex. Here, we found that E3 ubiquitin ligase SINAT5, an Arabidopsis homologue of the Drosophila SINA RING-finger protein, interacts directly with LHY, a component of the circadian oscillator, and DET1, a negative regulator of light-regulated gene expression. We also found that SINAT5 has E3 ubiquitination activity for LHY but not for DET1. Interestingly, LHY ubiquitination by SINAT5 was inhibited by DET1. Late flowering of sinat5 mutants indicates that flowering time can be controlled by DET1 through regulation of LHY stability by SINAT5.
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