10         20         30         40         50         60
         |          |          |          |          |          |
MAATVATAAA VAPAPAPGTD SASSVHWFRK GLRLHDNPAL LAAVRGARCV RCVYILDPWF - 60
AASSSVGINR WRFLLQSLED LDTSLRKLNS RLFVVRGQPA DVFPRLFKEW GVTRLTFEYD - 120
SEPFGKERDA AIMKMAKEAG VEVVTENSHT LYDLDRIIEL NGQKPPLTYK RFQAIISRME - 180
LPKKPVGLVT SQQMESCRAE IQENHDETYG VPSLEELGFP TEGLGPAVWQ GGETEALARL - 240
DKHLERKAWV ANYERPRMNA NSLLASPTGL SPYLRFGCLS CRLFYYRLWD LYKKVKRNST - 300
PPLSLFGQLL WREFFYTAAT NNPRFDRMEG NPICIQIPWD RNPEALAKWA EGKTGFPWID - 360
AIMTQLRQEG WIHHLARHAV ACFLTRGDLW VSWESGVRVF DELLLDADFS VNAGSWMWLS - 420
CSAFFQQFFH CYCPVGFGRR TDPSGDYIRR YLPKLKAFPS RYIYEPWNAP ESIQKAAKCI - 480
IGVDYPRPIV NHAETSRLNI ERMKQIYQQL SRYRGLCLLA SVPSCVEDLS HPVAEPSSSQ - 540
AGSMSSAGPR PLPSGPASPK RKLEAAEEPP GEELSKRARV AELPTPELPS KDA

Blue light has played a particularly important role as a driving force in evolution since it is the only component of the sunlight spectrum to penetrate to significant depths in aquatic environments, such as those in which life began on earth. Five classes of blue-light photoreceptors have been identified in bacteria, fungi, plants, and animals. These include the BLUF-domain proteins (AppA, PAC), PAS-domain proteins (PYP, wc-1), phototropins, UV/blue opsins, and photolyase/cryptochrome family. Of these blue-light photoreceptors, only the latter is shared by all of these diverse organisms. Cryptochromes were initially identified as putative photoreceptors because of their high degree of homology to the blue-light-activated DNA repair enzyme photolyase and the observation that, like photolyase, they contain two chromophores, a photoantenna pigment, folate, and the catalytic chromophore FAD. Photolyases catalyze the lightdependent repair of UV-induced cyclobutane pyrimidine dimers or pyrimidine-pyrimidone photoproducts in DNA, whereas cryptochromes lack DNA repair activity and act as photoreceptors for a variety of growth and adaptive responses, such as circadian rhythms and light-dependent transcriptional regulation. The photocycle of photolyase is well characterized. Photolyase binds UV-damaged DNA independently of light; absorption of a photon by the photoantenna MTHF is followed by resonance energy transfer to the catalytic chromophore FADH-, which splits the photoproduct by nonreductive electron transfer. Blue light-dependent regulator of the circadian feedback loop. Inhibits CLOCK|NPAS2-ARNTL E box-mediated transcription. Acts, in conjunction with CRY2, in maintaining period length and circadian rhythmicity. Has no photolyase activity. Capable of translocating circadian clock core proteins such as PER proteins to the nucleus. May inhibit CLOCK|NPAS2-ARNTL transcriptional activity through stabilizing the unphosphorylated form of ARNTL.