The developmental program of nodulation is regulated systemically in leguminous host species. A mutant astray (Ljsym77) in Lotus japonicus has lost some sort of its ability to regulate this symtem, and shows enhanced and early nodulation. In the absence of rhizobia, this mutant exhibits characteristics associated with defects in light and gravity responses. These nonsymbiotic phenotypes of astray are very similar to those observed in photomorphogenic Arabidopsis mutant hy5. Based on this evidence, we predicted that astray might contain a mutation in the HY5 homologue of L. japonicus. The homologue, named LjBzf, encodes a basic leucine zipper protein in the C-terminal half that shows the highest level of identity with HY5 of all Arabidopsis proteins. It also encodes legume-characteristic combination of motifs, including a RING-finger motif and an acidic region in the N-terminal half. The astray phenotypes were cosegregated with LjBzf, and the failure to splice the intron was detected. Nonsymbiotic and symbiotic phenotypes of astray were complemented by introduction of CaMV35SLjBzf. It is noteworthy that although Arabidopsis hy5 showed an enhancement of lateral root initiation, Lotus astray showed an enhancement of nodule initiation but not of lateral root initiation. Legume-characteristic combination of motifs of ASTRAY may play specific roles in the regulation of nodule development.