Characterization of the cis elements in the proximal promoter regions of the anthocyanin pathway genes reveals a common regulatory logic that governs pathway regulation

Cellular activities such as compound synthesis often require the transcriptional activation of an entire pathway; however, the molecular mechanisms underlying pathway activation have rarely been explained. Here, the cis regulatory architecture of the anthocyanin pathway genes targeted by the transcription factor (TF) complex including MYB, bHLH, and WDR was systematically analysed in one species and the findings extended to others. In Ipomoea purpurea, the IpMYB1-IpbHLH2-IpWDR1 (IpMBW) complex was found to be orthologous to the PAP1-GL3-TTG1 (AtPGT) complex of Arabidopsis thaliana, and interacted with a 7-bp MYB-recognizing element (MRE) and a 6-bp bHLH-recognizing element (BRE) at the proximal promoter region of the pathway genes. There was little transcription of the gene in the absence of the MRE or BRE. The cis elements identified experimentally converged on two syntaxes, ANCNNCC for MREs and CACN(A/C/T)(G/T) for BREs, and our bioinformatic analysis showed that these were present within anthocyanin gene promoters in at least 35 species, including both gymnosperms and angiosperms. For the anthocyanin pathway, IpMBW and AtPGT recognized the interspecific promoters of both early and later genes. In A. thaliana, the seed-specific TF complex (TT2, TT8, and TTG1) may regulate all the anthocyanin pathway genes, in addition to the proanthocyanidin-specific BAN. When multiple TF complexes in the anthocyanin pathway were compared, the cis architecture played a role larger than the TF complex in determining the variation in promoter activity. Collectively, a cis logic common to the pathway gene promoters was found, and this logic is essential for the trans factors to regulate the pathway.