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 st udies suggested that His-to-Asp phosphorelay mechanisms are involved presum ably in propagation of environmental stimuli, such as phytohormones (e.g. e thylene and cytokinin). Here we identified and characterized a set of novel Arabidopsis genes whose products considerably resemble the authentic respo nse regulators (ARR-series) of Arabidopsis in the sense that they have a ph ospho-accepting receiver-like domain. However, they should be discriminated from the classical ones in the strict sense that they lack the invariant p hospho-accepting aspartate site. They were thus named APRRs (Arabidopsis ps eudo-response regulators). Two such representatives, APRR1 and APRR2, were characterized extensively through cloning of the corresponding cDNAs, in te rms of their structural designs, biochemical properties, subcellular locali zation in plant cells, and expression profiles at the transcriptional level . The result of in vitro phosphorylation experiment with the Arabidopsis AH P phosphotransmitter suggested that the pseudo-receivers have no ability to undergo phosphorylation, The result of transient expression assay with oni on 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-m otif, appears to be responsible for the nuclear-localization, The most intr iguing result was that the accumulation of APRR1 transcript is subjected to a circadian rhythm. The APRR1 protein is identical to the one that was rec ently suggested to interact with the ABI3 (ABISCISIC ACID INSENSITIVE3) pro tein. These are discussed with special reference to the His-to-Asp phosphor elay signal transduction and circadian rhythm in Arabidopsis thaliana.