Identification of the residues in the Myb domain of maize C1 that specify the interaction with the bHLH cofactor R

The maize Myb transcription factor C1 depends on the basic helix-loop-helix (bHLH) proteins R or B for regulatory function, but the closely related My b protein P does not. We have used the similarity between the Myb domains o f C1 and P to identify residues that specify the interaction between the My b domain of C1 and the N-terminal region of R. Substitution of four predict ed solvent-exposed residues in the first helix of the second Myb repeat of P with corresponding residues from C1 is sufficient to confer on P the abil ity to physically interact with R. However, two additional Myb domain amino acid changes are needed to make the P regulatory activity partially depend ent on R in maize cells. Interestingly, when P is altered so that it intera cts with R, it can activate the Bz1 promoter, normally regulated by C1 + R but not by P. Together, these findings demonstrate that the change of a few amino acids within highly similar Myb domains can mediate differential int eractions with a transcriptional coregulator that plays a central role in t he regulatory specificity of C1, and that Myb domains play important roles in combinatorial transcriptional regulation.