J. M. Hernandez, T. J. Matulnik, and E. Grotewold

Department of Plant Biology and Plant Biotechnology Center, The Ohio State University, Columbus Ohio.

The maize Myb domain proteins C1 and P regulate the flavonoid biosynthetic pathway. Despite the fact that these transcription factors share over 75% identity in their R2R3 Myb motifs and that both of them activate the A1 gene, their regulatory specificity is clearly different. In addition, C1 requires a member of the bHLH-containing R/B gene family in order to activate the genes in the anthocyanin accumulation branch of the pathway while P is able to control the accumulation of 3-deoxy flavonoids and phlobaphene pigments without R/B. Given their high level of identity these two Myb proteins provide a unique opportunity to elucidate the mechanisms by which their regulatory specificity is achieved and how combinatorial interactions with other cellular factors participate in this regulation. Transient expression experiments using chimeric proteins between P and C1 allowed to identify regions involved in the regulatory specificity and in the interaction with R/B proteins. Based on the results obtained with these chimeric proteins, it was possible to identify specific residues responsible for some of these functions by site directed mutagenesis of the Myb domain of P and C1. These chimeric proteins also allowed the identification of Myb-domain residues responsible for the co-factor-dependent activity of C1, illustrating general mechanisms by which Myb-domain transcription factors may control gene expression. The involvement of other cellular factors necessary for the regulatory activity and biological specificity of P and C1 will be discussed.