Arabidopsis HY5 is a bZIP transcription factor that promotes photomorphogenesis. Previous studies suggested that COP1, a negative regulator of photomorphogenesis, directly interacts with nuclear HY5 and targets it for proteasome-mediated degradation. Light negatively regulates the nuclear level of COP1 and thus permits HY5 accumulation. Here we report that HY5 abundance peaks in early seedling development, consistent with its role in promoting photomorphogenesis. HY5 acts exclusively within a complex and exists in two isoforms, resulting from phosphorylation within its COP1 binding domain by a lightregulated kinase activity. Unphosphorylated HY5 shows stronger interaction with COP1, is the preferred substrate for degradation, has higher af®nity to target promoters and is physiologically more active than the phosphorylated version. Therefore, HY5 phosphorylation provides an added level of lightmediated regulation of HY5 stability and activity besides nuclear COP1 levels. Regulated HY5 phosphorylation not only provides abundant and physiologically more active unphosphorylated HY5 in the light, but also helps to maintain a small pool of less active phosphorylated HY5 in the dark, which could be essential for a rapid initial response during darkto-light transition.

SummaryExperiments with gene-trap vectors containing the ®re¯y luciferase (LUC ) reporter genes were carried out with the aim of analyzing functions of the Arabidopsis genome. Studies with protein fusion-type trap vectors as well as an internal ribosome entry site (IRES)-assisted non-fusion-type vector revealed that both types of vectors were suitable for gene trapping in Arabidopsis, although there were some differences in trapping ef®ciencies. The established trap lines were subjected to analyses for light responses, demonstrating the powerful and unique applications of a LUC-trapping system. A systematic survey of the insertion sites of the T-DNAs in LUC-expressing lines revealed 12±41% gene-trapping ef®ciencies depending on the vector. We demonstrate that the LUC-trapping system provides a unique system with which to monitor temporal expression of plant genes.

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Acid-catalyzed chemical equilibration studies on the titled compounds have been carried out. The β,γ-isomers are highly favored at equilibrium in the cases of β-chloro- and β-phenylthio-substituted ketones. Hydrogen-bonding between carbonyl oxygen and sulfonium or amine proton stabilizes the α,β-unsaturated form.

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