Extensive mutagenesis of a transcriptional activation domain identifies single hydrophobic and acidic amino acids important for activation in vivo

CI is a transcriptional activator of genes encoding biosynthetic enzymes of the maize anthocyanin pigment pathway. CI has an amino terminus homologous to Myb DNA-binding domains and an acidic carboxyl terminus that is a transcriptional activation domain in maize and yeast cells. To identify amino acids critical for transcriptional activation, an extensive random mutagenesis of the CI carboxyl terminus was done. The Cl activation domain is remarkably tolerant of amino acid substitutions, as changes at 34 residues had little or no effect on transcriptional activity. These changes include introduction of helix-incompatible amino acids throughout the Cl activation domain and alteration of most single acidic amino acids, suggesting that a previously postulated amphipathic cr-helix is not required for activation. Substitutions at two positions revealed amino acids important for transcriptional activation. Replacement of leucine 253 with a proline or glutamine resulted in approximately 10% of wild-type transcriptional activation. Leucine 253 is in a region of Cl in which several hydrophobic residues align with residues important for transcriptional activation by the herpes simplex virus VP16 protein. However, changes at all other hydrophobic residues in Cl indicate that none are critical for Cl transcriptional activation. The other important amino acid in Cl is aspartate 262, as a change to valine resulted in only 24% of wild-type transcriptional activation. Comparison of our Cl results with those from VP16 reveal substantial differences in which amino acids are required for transcriptional activation in vivo by these two acidic activation domains.