BACKGROUND: The circadian expression of the mammalian clock genes is based on 
transcriptional feedback loops. Two basic helix-loop-helix (bHLH) PAS (for 
Period-Arnt-Sim) domain-containing transcriptional activators, CLOCK and BMAL1, 
are known to regulate gene expression by interacting with a promoter element 
termed the E-box (CACGTG). The non-canonical E-boxes or E-box-like sequences 
have also been reported to be necessary for circadian oscillation.
RESULTS: We report a new cis-element required for cell-autonomous circadian 
transcription of clock genes. This new element consists of a canonical E-box or 
a non-canonical E-box and an E-box-like sequence in tandem with the latter with 
a short interval, 6 base pairs, between them. We demonstrate that both E-box or 
E-box-like sequences are needed to generate cell-autonomous oscillation. We also 
verify that the spacing nucleotides with constant length between these 2 
E-elements are crucial for robust oscillation. Furthermore, by in silico 
analysis we conclude that several clock and clock-controlled genes possess a 
direct repeat of the E-box-like elements in their promoter region.
CONCLUSION: We propose a novel possible mechanism regulated by double E-box-like 
elements, not to a single E-box, for circadian transcriptional oscillation. The 
direct repeat of the E-box-like elements identified in this study is the minimal 
required element for the generation of cell-autonomous transcriptional 
oscillation of clock and clock-controlled genes.