In petunia flowers, the loci anl, an2, and anll control the pigmentation of the flower by stimulating the transcription of anthocyanin biosynthetic genes. The anl and an2 locus were recently cloned and encode a basic helix-loop-helix (bHLH) and MYB-domain transcriptional activator, respectively. Here, we report the isolation of the anll locus by transposon tagging. RNA gel blot experiments show that anll is expressed independently from anl and an2 throughout plant development, as well as in

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... ues that do not express the anthocyanin pathway. It encodes a novel WD-repeat protein that is highly conserved even in species that do not produce anthocyanins such as yeast, nematodes, and mammals. The observation that the human anll homolog partially complements the anll petunia mutant in transient assays shows that sequence similarity reflects functional conservation. Overexpression of an2 in anll-petals restored the activity of a structural anthocyanin gene in transient assays, indicating that AN11 acts upstream of AN2. Cell fractionation experiments show that the bulk of the ANll protein is localized in the cytoplasm. Taken together, this indicates that ANll is a cytoplasmic component of a conserved signal transduction cascade that modulates AN2 function in petunia, thereby linking cellular signals with transcriptional activation.