Ubiquitin is a small polypeptide (10kDa) and ubiquitination is one of the post-translational modifications (PTMs) by ubiquitin protein, resulting in degradation of the target proteins by 26S proteasome complex. Here, we found that E3 ubiquitin ligase SINAT5, an Arabidopsis homolog of the Drosophila SINA RING-finger protein, directly interacts with LHY (late elongated hypocotyls), a component of the circadian oscillator, and DET1 (De-etiolated 1), a negative regulator of light-regulated gene expression. In addition, we also found that SINAT5 has an E3 ubiquitination activity for LHY but not for DET1. Interestingly, LHY ubiquitination by SINAT5 was inhibited by DET1, suggesting that flowering time through regulation of LHY stability by SINAT5 may be controlled by DET1. We are now investigating whether DET1 indeed involves in the regulation of the proteolytic turnover of the LHY by SINAT5 in vivo.