[Show abstract][Hide abstract] ABSTRACT: Cryptochrome (Cry) photoreceptors share high sequence and structural similarity with DNA repair enzyme DNA-photolyase and
carry the same flavin cofactor. Accordingly, DNA-photolyase was considered a model system for the light activation process
of cryptochromes. In line with this view were recent spectroscopic studies on cryptochromes of the CryDASH subfamily that
showed photoreduction of the flavin adenine dinucleotide (FAD) cofactor to its fully reduced form. However, CryDASH members
were recently shown to have photolyase activity for cyclobutane pyrimidine dimers in single-stranded DNA, which is absent
for other members of the cryptochrome/photolyase family. Thus, CryDASH may have functions different from cryptochromes. The
photocycle of other members of the cryptochrome family, such as Arabidopsis Cry1 and Cry2, which lack DNA repair activity but control photomorphogenesis and flowering time, remained elusive. Here we
have shown that Arabidopsis Cry2 undergoes a photocycle in which semireduced flavin (FADH·) accumulates upon blue light irradiation. Green light irradiation of Cry2 causes a change in the equilibrium of flavin oxidation
states and attenuates Cry2-controlled responses such as flowering. These results demonstrate that the active form of Cry2
contains FADH· (whereas catalytically active photolyase requires fully reduced flavin (FADH-)) and suggest that cryptochromes could represent photoreceptors using flavin redox states for signaling differently from
DNA-photolyase for photorepair.
[Show abstract][Hide abstract] ABSTRACT: Plants have several blue-light receptors, which regulate different aspects of growth and development. Recent studies have identified three such receptors: cryptochrome 1, cryptochrome 2 and phototropin. Cryptochromes 1 and 2 are photolyase-like receptors that regulate hypocotyl growth and flowering time; phototropin mediates phototropism in response to blue light. In addition, phytochrome A has also been found to mediate various blue-light responses. Although the signal-transduction mechanisms of blue-light receptors remain largely unclear, phototropin is probably a protein kinase that regulates cytoplasmic calcium concentrations, whereas the cryptochromes might regulate anion-channel activity and changes in gene expression.