Detailed structure/function analysis of the C-terminal region of mammalian CRY1 allowed us to identify a putative coiled-coil domain at the beginning of the C-terminal extension as a potential PER and BMAL1 binding site. Deletion of the complete Cterminal extension abolished the CLOCK/BMAL1 transcription inhibitory potential of CRY1, Similarly, Green and co-workers have demonstrated that the Cterminal extension of Xenopus laevis CRY proteins is crucial for transcription repression. Interestingly, specific deletion of either the coiled-coil domain or the downstream tail region of mammalian CRY1 failed to eliminate its ability to inhibit CLOCK/BMAL1-mediated transcription , likely because these mutant proteins can still bind to CLOCK via a yet unidentified region of the core domain. This finding lead us to suggest that an interaction between the C-terminal extension and the core domain is mandatory for the clock function of mammalian cryptochromes, possibly by providing structure to the latter. In the LDK378 present study, we have explored the importance of the core domain of mammalian CRY proteins for core oscillator function by addressing the question to what extent photolyase enzymes affect circadian core oscillator function. Using in vivo and in vitro approaches, we show that Potorous tridactylus CPD photolyase not only displays cryptochrome-associated functions, but also can replace the CRY proteins in the mammalian circadian core oscillator. The construct used to generate the Per2::Luc transgenic mice consists of the luciferase gene under control of the mPer2 promoter, cloned in pBS. The primers used to amplify the 4.2 kb mPer2 Adriamycin abmole bioscience promoter fragment are indicated in Figure S1. Intronic sequences from the rabbit b-globin locus were included in the expression construct for messenger stability. The expression construct fragment was excised from the plasmid using appropriate restriction enzymes, separated from the vector DNA by agarose gel electrophoresis, isolated from the gel with the GeneClean II kit , and further purified using Elutip-Dmini columns. The fragment was dissolved in injection buffer and injected in the pronucleus of fertilized eggs derived from FVB/N intercrosses as described. Animals were backcrossed in a C57BL/6J background. Genotyping was performed by PCR using primers located in the luciferase gene. Annealing was performed at 55uC. DNA derived from transgenic mice rendered a PCR product of 475 bp, whereas no product was detected using DNA from wild type litter mates. As photolyases structurally resemble cryptochromes , and given the observation that transient constitutive overexpression of the CRY1 protein suppresses the rhythmic expression of a cotransfected Bmal1 Luc reporter gene , we next investigated the effect of transient overexpression of PtCPD-PL on the circadian clock of cultured fibroblasts.