Genes Encoding Pseudo-Response Regulators: Insight into His-to-Asp Phosphorelay and Circadian Rhythm in Arabidopsis thaliana

In the higher plant, Arabidopsis thaliana, results from recent intensive studies suggested that His-to-Asp phosphorelay mechanisms are involved presumably in propagation of environmental stimuli, such as phytohormones (e.g. ethylene and cytokinin). Here we identified and characterized a set of novel Arabidopsis genes whose products considerably resemble the authentic response regulators (ARR-series) of Arabidopsis in the sense that they have a phospho-accepting receiver-like domain. However, they should be discriminated from the classical ones in the strict sense that they lack the invariant phospho-accepting aspartate site. They were thus named APRRs (Arabidopsis pseudo-response regulators). Two such representatives, APRR1 and APRR2, were characterized extensively through cloning of the corresponding cDNAs, in terms of their structural designs, biochemical properties, subcellular localization in plant cells, and expression profiles at the transcriptional level. The result of in vitro phosphorylation experiment with the Arabidopsis AHP phosphotransmitter suggested that the pseudo-receivers have no ability to undergo phosphorylation. The result of transient expression assay with onion epidermal cells showed that the GFP-APRR1 fusion protein has an ability to enter into the nuclei. The C-terminal domain of APRR1, termed CONSTANS-motif, appears to be responsible for the nuclear-localization. The most intriguing result was that the accumulation of APRR1 transcript is subjected to a circadian rhythm. The APRR1 protein is identical to the one that was recently suggested to interact with the ABI3 (ABISCISIC ACID INSENSITIVE3) protein. These are discussed with special reference to the His-to-Asp phosphorelay signal transduction and circadian rhythm in Arabidopsis thaliana.