Abstract

SPECIFIC responses to blue light are found throughout the biological kingdom. These responses& mdash;which in higher plants include photo-tropism, inhibition of hypocotyl elongation, and stomatal opening¹& mdash;are in many cases thought to be mediated by flavin-type photoreceptors². But no such blue-light photoreceptor has yet been identified or isolated, although blue-light responses in plants were reported by Darwin over a century ago³, long before the discovery of the now relatively well characterized red/far-red light photoreceptor, phytochrome⁴. Here we describe the isolation of a gene corresponding to the HY4 locus of Arabidopsis thaliana. The hy4 mutant⁵ is one of several mutants⁶ that are selectively insensitive to blue light during the blue-light-dependent inhibition of hypocotyl elongation response, which suggests that they lack an essential component of the cryptochrome-associated light-sensing pathway. The HY4 gene, isolated by gene tagging, was shown to encode a protein with significant homology to microbial DNA photolyases. As photolyases are a rare class of flavoprotein that catalyse blue-light-dependent reactions⁷, the protein encoded by HY4 has a structure consistent with that of a flavin-type blue-light photoreceptor.