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Abstract
Specific responses to blue light are found throughout the biological kingdom. These
responses--which in higher plants include phototropism, inhibition of hypocotyl elongation,
and stomatal opening--are in many cases thought to be mediated by flavin-type photoreceptors.
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 ago, long before
the discovery of the now relatively well characterized red/far-red light photoreceptor,
phytochrome. Here we describe the isolation of a gene corresponding to the HY4 locus
of Arabidopsis thaliana. The hy4 mutant is one of several mutants 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 reactions, the protein
encoded by HY4 has a structure consistent with that of a flavin-type blue-light photoreceptor.