Studies on Molecular Mechanism of Abscisic Acid-Regulated Transcription

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The rice basic region-leucine zipper (bZIP) factor TRAB1 binds to abscisic acid response elements (ABREs) and mediates abscisic acid (ABA) signals to activate transcription. We show here that TRAB1is rapidly phosphorylated by in vivo labeling experiments and by phosphatase-sensitive mobilty-shifts on a SDS-polyacrylamide gel. We previously have shown that a chimeric promoter containing GAL4-binding sites becomes ABA-inducible when a GAL4-binding domain : : TRAB1 fusion protein is expressed. This expression system allowed us to assay the ABA-response function of TRAB1. Using this assay system, we have identified that Ser-44 of TRAB1 is critical for this function. Since ABA-induced mobility-shift was not observed when Ser-44 was mutated to alanine, this serine residue is suggested to be phosphorylated in response to ABA. Cell-fractionation experiments as well as the fluorescent microscopic observation of transiently expressed GFP : : TRAB1 fusion protein indicated that TRAB1 is localized in the nucleus independently of ABA. Our results suggest that the end or near-end point event of the primary ABA signal transduction pathway is the phosphorylation in the nucleus of pre-existing TRAB1 and probablly other homologous ABRE -binding factors.