The FAR1 locus encodes a novel nuclear protein specific to phytochrome A signaling

The phytochrome family of photoreceptors has a well-defined role in regulating gene expression in response to informational light signals. Little is known, however, of the early steps of phytochrome signal transduction. Here we describe a new Arabidopsis mutant, far1 (far-red-impaired response), which has reduced responsiveness to continuous far-red light, but responds normally to other light wavelengths. This phenotype implies a specific requirement for FAR1 in phyA signal transduction. The far1 locus maps to the south arm of chromosome 4, and is not allelic to photomorphogenic loci identified previously. All five far1 alleles isolated have single nucleotide substitutions that introduce stop codons in a single ORF. The I;ARI gene encodes a protein with no significant sequence similarity to any proteins of known function. The FAR1 protein contains a predicted nuclear localization signal and is targeted to the nucleus in transient transfection assays. This result supports an emerging view that early steps in phytochrome signaling may be centered in the nucleus. The FAR1 gene defines a new multigene family, which consists of at least four genes in Arabidopsis. This observation raises the possibility of redundancy in the phyA-signaling pathway, which could account for the incomplete block of phyA signaling observed in the far1 mutant.