The interaction domains of the plant Myc-like bHLH transcription factors can regulate the transactivation strength

The N-terminal region of the plant Myc-like basic helix-loop-helix transcription factors (bHLH TFs) contains two domains. Approximately, 190 amino acids at the N-terminus comprise an interaction domain, a.k.a. Myb-interacting-region (MIR) for its primary function of interacting with Myb-like TFs. Following, the interaction domain is an activation (or acidic) domain responsible for transactivation. We have previously discovered that a lysine to methionine substitution (K157M) in the interaction domain of Myc-RP of Perilla frutescens leads to a 50-fold increase in transactivation activity. The result suggests that mutations in the interaction domain affect transactivation. The highly conserved nature of this lysine residue in many Myc-like bHLH TFs prompted us to explore the functional importance of this residue within the TF family and the influence of the interaction domain on the activation domain in transactivation. We found that the replacement of the equivalent lysine with methionine significantly affects the transactivation activities of two other Myc-RP homologues, Delila from snapdragon and Lc from maize. In addition to methionine, substitution with several other amino acids at this position has positive effects on transcriptional activity. A neighboring conserved alanine residue (A159 in Myc-RP, A161 in Delila and A172 in Lc) also affects transactivation. Substitution of this alanine residue to an aspartic acid abolished transactivation of both Myc-RP and Delila and severely reduced transactivation of Lc. Ectopic expression of a Myc-RP K157M mutant in transgenic tobacco resulted in increased anthocyanin accumulation compared to plants expressing the wild-type gene. Our study reveals the potential cooperation between functional domains of the bHLH TFs.