Edited by Joel M. Gottesfeld The mammalian CLOCK:BMAL1 transcription factor complex and its coactivators CREB-binding protein (CBP)/p300 and mixed-lineage leukemia 1 (MLL1) critically regulate circadian transcription and chromatin modification. Circadian oscillations are regulated by interactions of BMAL1's C-terminal transactivation domain (TAD) with the KIX domain of CBP/p300 (activating) and with the clock protein CRY1 (repressing) as well as by the BMAL1 G-region preceding the TAD. Circadian acetylation of Lys 537 within the G-region enhances repressive BMAL1-TAD-CRY1 interactions. Here, we characterized the interaction of the CBP-KIX domain with BMAL1 proteins, including the BMAL1-TAD, parts of the G-region, and Lys 537. Tethering the small compound 1-10 in the MLL-binding pocket of the CBP-KIX domain weakened BMAL1 binding, and MLL1-bound KIX did not form a ternary complex with BMAL1, indicating that the MLL-binding pocket is important for KIX-BMAL1 interactions. Small-angle X-ray scattering (SAXS) models of BMAL1 and BMAL1:KIX complexes revealed that the N-terminal BMAL1 G-region including Lys 537 forms elongated extensions emerging from the bulkier BMAL1-TAD:KIX core complex. Fitting high-resolution KIX domain structures into the SAXS-derived envelopes suggested that the G-region emerges near the MLL-binding pocket, further supporting a role of this pocket in BMAL1 binding. Additionally, mutations in the second CREB-pKID/c-Myb-binding pocket of the KIX domain moderately impacted BMAL1 binding. The BMAL1(K537Q) mutation mimicking Lys 537 acetylation, however, did not affect the KIX-binding affinity, in contrast to its enhancing effect on CRY1 binding. Our results significantly advance the mechanistic understanding of the protein interaction networks controlling CLOCK:BMAL1-and CBP-dependent gene regulation in the mammalian circadian clock. Most organisms possess circadian rhythms coordinated by circadian clocks that synchronize them to the 24-h light-dark cycle. In mammals, circadian rhythmicity is controlled by transcriptional/translational feedback loops. The core transcriptional/translational feedback loop consists of the heterodimeric basic helix-loop-helix (bHLH) 4-PER-ARNT-SIM (PAS) domaincontaining transcriptional activators CLOCK (circadian locomotor output cycle kaput) and BMAL1 (brain and muscle ARNT-like protein-1), which circadianly regulate genes encoding Period (PER1, PER2, and PER3) and Cryptochrome (CRY1 and CRY2) clock proteins as well as many clock-controlled genes (1, 2). Analyses in mouse liver led to a model of three temporally separated states of CLOCK:BMAL1-dependent circadian transcription: an early repressive state, a late repressive state, and an active state (3). Large multisubunit PER-and CRY-containing complexes interact with CLOCK:BMAL1 in the early phase of repression (4), whereas CRY1 alone maintains the late repressive state (5, 6). In the active state, CLOCK:BMAL1 associates with co-activators such as the histone acetyl transferase CREB-binding protein (CBP) and its paralo...