Arabidopsis ARR1 and ARR2 response regulators operate as transcriptional activators

The genes coding for the response regulators ARR1 and ARR2 have previously been identified by in silico screening of an expression sequence tag databa se and subsequent cloning from both Arabidopsis cDNA and genomic libraries. Their structures, in which the N-terminal signal receiver domain is follow ed by the output domain, are characteristic of typical bacterial response r egulators of the two-component regulatory systems that control responses to a variety of environmental stimuli. Here we present evidence that these re sponse regulators actually work as transcription factors. ARR1 and ARR2 wer e localized in the nuclei of plant cells regardless of the presence or abse nce of their signal receiver domain. Their middle segments, which faintly r esemble the mammalian oncogene product Myb, were capable of binding double- stranded DNA in a sequence-specific manner in vitro. Their C-terminal halve s functioned as transactivation domains in plant cells when combined with t he DNA-binding domain of yeast GAL4. They thus possess all the essential co mponents of a transcriptional activator. Both ARR1 and ARR2 promoted expres sion of a reporter gene in plant cells through their own target sequence. T runcation of their N-terminal signal receiver domain led to an increase in transactivation. An as yet unidentified phospho-relay signal may modulate t he capability for transactivation and/or DNA binding through the signal rec eiver domain.