anthocyanin1 of petunia encodes a basic helix-loop-helix protein that directly activates transcription of structural anthocyanin genes

The petunia loci anthocyanin1 (an1), an2, an4, and an11 are required for th e transcription of anthocyanin biosynthetic genes in floral organs. The an2 and an11 loci were recently cloned and shown to encode a MYB-domain transc riptional activator and a cytosolic WD40 protein, respectively. Here, we re port the isolation of anl by transposon tagging. anl encodes a new member o f the basic helix-loop-helix family of transcription factors that is functi onally and evolutionarily distinct from JAF13, the apparent petunia ortholo g of maize RED1 and snapdragon DELILA. We provide genetic evidence that the transcription factors encoded by anl, an2, and an4 operate in an unexpecte dly complex regulatory hierarchy. In leaves, ectopic expression of AN2 indu ces anl expression, whereas in anthers, anl expression depends on an4, enco ding (or controlling) a MYB protein that is paralogous to AN2. Experiments with transgenic plants expressing a post-translationally controlled AN1-GLU COCORTICOID RECEPTOR fusion protein indicated that independent of protein s ynthesis, AN1 directly activates the expression of the dfrA gene encoding t he enzyme dihydroflavonol 4-reductase and of Pmyb27 encoding a MYB-domain p rotein of unknown function.