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

The inducible two-component signal transduction mechanism typically involves 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 variety of cellular activities in response to environmental stimuli. Recently, several components of this signaling mechanism including sensor kinases and response regulators have been identified in eukaryotes, including plants. Here, we describe a new subclass of eukaryotic Arabidopsis response regulator-like protein (ARLP) genes that are differentially expressed in plant tissues. The receiver modules of the encoded proteins contain the amino acid residues conserved in all response regulators and have a predicted response regulator-like secondary structure. These novel ARLPs are localized in the nucleus, are transcriptionally active in yeast when fused to the heterologous Gal4 DNA-binding domain and, within their output domains, show structural homology to basic helix-loop-helix (bHLH) transcription factors. These results strongly suggest that response regulators with transcription factor properties exist in higher plants.