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

The phytochrome family of photoreceptors has a well-defined role in regulat ing gene expression in response to informational light signals. Little is k nown, however, of the early steps of phytochrome signal transduction. Here we describe a new Arabidopsis mutant, far1 (far-red-impaired response), whi ch has reduced responsiveness to continuous far-red light, but responds nor mally to other light wavelengths. This phenotype implies a specific require ment for FAR1 in phyA signal transduction. The far1 locus maps to the south arm of chromosome 4, and is not allelic to photomorphogenic loci identifie d previously. All five far1 alleles isolated have single nucleotide substit utions 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 f unction. The FAR1 protein contains a predicted nuclear localization signal and is targeted to the nucleus in transient transfection assays. This resul t supports an emerging view that early steps in phytochrome signaling may b e centered in the nucleus. The FAR1 gene defines a new multigene family, wh ich consists of at least four genes in Arabidopsis. This observation raises the possibility of redundancy in the phyA-signaling pathway, which could a ccount for the incomplete block of phyA signaling observed in the far1 muta nt.