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

Abstract

Tissue-specific expression of the maize anthocyanin Bronze-1 (Bz1) gene is controlled by the products of several regulatory genes. These include C1 or Pl 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 oligonucleotide 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.