Differential expression and nuclear localization of response regulator-like proteins from Arabidopsis thaliana

The inducible two-component signal transduction mechanism typically involve s two common signaling molecules. First, a sensor kinase that consists of a signal-sensing input domain physically coupled to a transmitter module and , second, a response regulator that contains a receiver module and a signal -output domain. Such sensor kinase/response regulator pairs trigger a varie ty of cellular activities in response to environmental stimuli. Recently, s everal components of this signaling mechanism including sensor kinases and response regulators have been identified in eukaryotes, including plants. H ere, we describe a new subclass of eukaryotic Arabidopsis response regulato r-like protein (ARLP) genes that are differentially expressed in plant tiss ues. The receiver modules of the encoded proteins contain the amino acid re sidues conserved in all response regulators and have a predicted response r egulator-like secondary structure. These novel ARLPs are localized in the n ucleus, are transcriptionally active in yeast when fused to the heterologou s Gal4 DNA-binding domain and, within their output domains, show structural homology to basic helix-loop-helix (bHLH) transcription factors. These res ults strongly suggest that response regulators with transcription factor pr operties exist in higher plants.