Transcription factor involved in the circadian clock and in the phytochrome regulation. Binds to the promoter regions of APRR1/TOC1 and TCP21/CHE to repress their transcription. Binds to the promoter regions of CAB2A and CAB2B to promote their transcription. Represses both LHY and itself. Recognizes the promoter of JMJ14 to regulates its expression during the night in a circadian manner (PubMed:31429787). Binds to the promoter of BHLH80/FBH1 and regulates its expression in response to warm temperature, thus contributing to temperature compensation of the circadian clock (PubMed:25246594). Homodimer or heterodimer with LHY. Interacts with LHY; independently of photoperiod. Probably part of a large complex. Interacts with CKB1, CKB2 and CKB3. Interacts with LNK1 and LNK2 (PubMed:25012192). Moves into the nucleus very soo after translation. This translocation is unaffected by light and dark. Expressed in leaves, roots, stems, flowers and siliques. Circadian-regulation with peak levels occurring around 1 hour after dawn. Up-regulated by APRR1/TOC1 and transiently by light treatment. Down-regulated by APRR5, APRR7 and APRR9. The CCA1 mRNA is relatively stable in the dark and in far-red light but has a short half-life in red and blue light. Modulated by BHLH80/FBH1 via direct negative promoter regulation in response to warm temperature (at 28 degrees Celsius) (PubMed:25246594). The N-terminal part (11-82) is essential for DNA binding, an internal domain (136-316) is important for homodimerization and for interaction with LHY and the C-terminal part (317-608) is necessary for interaction with CKB3. Phosphorylated at Ser-5, Ser-6, and at least at one of the position Ser-431, Ser-432, Ser-433 or Ser-484. Phosphorylation is involved in dimerization and is necessary for binding to promoters and regulation of the circadian clock. Altered phytochrome regulation and shorter circadian oscillations (PubMed:10097183, PubMed:19218364). Abnormal pace and rhythmicity of the circadian clock (PubMed:25246594). The double mutant cca1 lhy accumulates higher levels of JMJ14 but lower levels of ATXR3/SDG2, and exhibits damped H3K4me3 levels near the transcription start sites of genic regions (PubMed:31429787). CCA1 and LHY are only partially redundant, but they bind to the same region of the promoters.