1 7 1 8 Word count: 5057 1 9 3 1 formerly CYP72B1) and SOB7 (CYP72C1) are two BR-inactivating cytochrome P450s 3 2 identified in Arabidopsis thaliana. We previously found that a TF ATAF2 (ANAC081) 3 3 suppresses BAS1 and SOB7 expression by binding to the Evening Element (EE) and CCA1-3 4 binding sites (CBS) on their promoters. Both EE and CBS are known binding targets of the 3 5 core circadian clock regulatory protein CCA1. Here, we confirm that CCA1 binds the EE 3 6 and CBS motifs on BAS1 and SOB7 promoters, respectively. Elevated accumulations of 3 7 BAS1 and SOB7 transcripts in the CCA1 null mutant cca1-1 indicate that CCA1 is a 3 8 repressor of their expression. When compared to either cca1-1 or the ATAF2 null mutant 3 9 ataf2-2, the cca1-1 ataf2-2 double mutant shows higher SOB7 transcript accumulations and 4 0 stronger BR-insensitive phenotype of hypocotyl elongation in white light. CCA1 interacts 4 1 with ATAF2 at both DNA-protein and protein-protein levels. ATAF2, BAS1 and SOB7 are 4 2 all circadian-regulated with distinct expression patterns. These results demonstrate that 4 3 CCA1 and ATAF2 differentially suppress BAS1-and SOB7-mediated BR inactivation. 4 4 4 5
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