C1- and R-dependent expression of the maize Bz1 gene requires sequences with homology to mammalian myb and myc binding sites

Tissue-specific expression of the maize anthocyanin Bronze-1 (Bz1) gene is controlled by the products of several regulatory genes. These include C1 or PI and R or B that share homology to the myb proto-oncogenes and myc-like genes respectively. Bz1 expression in embryo tissues is dependent on C1 and an R-sc allele of R. Transient expression from mutated and deleted versions of the Bz1 promoter fused to a luciferase reporter gene was measured in C1, Rscm2 embryos after gene transfer by microprojectiles. This analysis revealed that the sequences between -76 base pairs (bp) and -45 bp and a 9-bp AT-rich block between -88 bp and -80 bp were critical for Bz1 expression The -76 bp to -45 bp region includes two short sequences that are homologous to the consensus binding sites of the myb- and myc-like proteins. Site-specific mutations of these "myb" and "myc" sequences reduced Bz1 expression to 10% and 1% of normal, respectively. Additionally, a trimer of a 38-bp ohgonucleotide containing these myb and myc sites increased the expression of a cauliflower mosaic virus 35S minimal promoter by 26-fold This enhancement was dependent on both C1 and R. Because the sites critical for Bz1 expression are homologous to the myb and myc consensus binding sequences and the C1 and R proteins share homology with the myb and myc products, respectively, we propose that C1 and R interact with the Bz1 promoter at these sites.