In the present study, we investigated the 3' untranslated region (UTR) of the 
mouse core clock gene cryptochrome 1 (Cry1) at the post-transcriptional level, 
particularly its translational regulation. Interestingly, the 3'UTR of Cry1 mRNA 
decreased its mRNA levels but increased protein amounts. The 3'UTR is widely 
known to function as a cis-acting element of mRNA degradation. The 3'UTR also 
provides a binding site for microRNA and mainly suppresses translation of target 
mRNAs. We found that AU-rich element RNA binding protein 1 (AUF1) directly binds 
to the Cry1 3'UTR and regulates translation of Cry1 mRNA. AUF1 interacted with 
eukaryotic translation initiation factor 3 subunit B and also directly 
associated with ribosomal protein S3 or ribosomal protein S14, resulting in 
translation of Cry1 mRNA in a 3'UTR-dependent manner. Expression of cytoplasmic 
AUF1 and binding of AUF1 to the Cry1 3'UTR were parallel to the circadian CRY1 
protein profile. Our results suggest that the 3'UTR of Cry1 is important for its 
rhythmic translation, and AUF1 bound to the 3'UTR facilitates interaction with 
the 5' end of mRNA by interacting with translation initiation factors and 
recruiting the 40S ribosomal subunit to initiate translation of Cry1 mRNA.