Response of the Sensory Animal-like Cryptochrome aCRY to Blue and Red Light As Revealed by Infrared Difference Spectroscopy

Cryptochromes act as blue light sensors in plants, insects, fungi, and bacteria. Recently, an animal-like cryptochrome (aCRY) was identified in the green alga <i>Chlamydomonas reinhardtii</i> by which gene expression is altered in response to not only blue light but also yellow and red light. This unique response of a flavoprotein <i>in vivo</i> has been attributed to the fact that the neutral radical of the flavin chromophore acts as dark form of the sensor, which absorbs in almost the entire visible spectral range (<680 nm). Here, we investigated light-induced processes in the protein moiety of full-length aCRY by UV–vis and Fourier transform infrared spectroscopy. Findings are compared to published results on the homologous (6-4) photolyases, DNA repair enzymes. The oxidized state of aCRY is converted to the neutral radical by blue light. The recovery is strongly dependent on pH and might be catalyzed by a conserved histidine of the (6-4)/clock cluster. The decay is independent of oxygen concentration in contrast to that of other cryptochromes and (6-4) photolyases. This blue light reaction of the oxidized flavin is not accompanied by any detectable changes in secondary structure, in agreement with a role <i>in vivo</i> of an unphysiological preactivation. In contrast, the conversion by red light of the neutral radical to the anionic fully reduced state proceeds with conformational changes in turn elements, which most probably constitute a part of the signaling process. These changes have not been detected in the corresponding transition of (6-4) photolyase, which points to a decisive difference between the sensor and the enzyme