The response regulator ARR2: a pollen-specific transcription factor involved in the expression of nuclear genes for components of mitochondrial Complex I in Arabidopsis

Two-component signal systems regulate a variety of cellular activities. The y : Involve at least two common signalling molecules: a signal-sensing kina se and a response regulator that mediates the output response. Multistep sy stems also require proteins containing phosphotransfer domains. Here we rep ort that the response regulator ARR2 from Arabiciopsis is predominantly exp ressed in pollen and is localized in the nuclear compartment of the plant c ell. Furthermore, ARR2 is transcriptionally active in yeast and binds to th e promoters of nuclear genes for several components of mitochondrial respir atory chain complex I (nCI) from Arabidopsis. The nuclear nCI genes are up- regulated in pollen during spermatogenesis. The transcription factor functi ons of ARR2 are mediated by its C-terminal output domain. Our data identify ARR2 as the first eukaryotic response regulator which functions as a trans cription factor at a known promoter sequence. Yeast two-hybrid analysis and in vitro interaction studies suggest that ARR2 very probably forms part of a multistep two-component signalling mechanism that includes HPt proteins like AHP1 or AHP2. These findings point to an as yet unidentified signal tr ansduction system that may regulate aspects of floral and mitochondrial gen e expression.