HY4 GENE OF A-THALIANA ENCODES A PROTEIN WITH CHARACTERISTICS OF A BLUE-LIGHT PHOTORECEPTOR

SPECIFIC responses to blue light are found throughout the biological k ingdom. These responses-which in higher plants include phototropism, i nhibition of hypocotyl elongation, and stomatal opening1-are in many c ases thought to be mediated by flavin-type photoreceptors2. But no suc h blue-light photoreceptor has yet been identified or isolated, althou gh blue-light responses in plants were reported by Darwin over a centu ry ago3, long before the discovery of the now relatively well characte rized red/far-red light photoreceptor, phytochrome. Here we describe t he isolation of a gene corresponding to the HY4 locus of Arabidopsis t haliana. The hy4 mutant5 is one of several mutants6 that are selective ly insensitive to blue light during the blue-light-dependent inhibitio n of hypocotyl elongation response, which suggests that they lack an e ssential component of the cryptochrome-associated light-sensing pathwa y. The HY4 gene, isolated by gene tagging, was shown to encode a prote in with significant homology to microbial DNA photolyases. As photolya ses are a rare class of flavoprotein that catalyse blue-light-dependen t reactions7, the protein encoded by HY4 has a structure consistent wi th that of a flavin-type blue-light photoreceptor.