- PH4 of Petunia Is an R2R3 MYB Protein That Activates Vacuolar Acidification through Interactions with Basic-Helix-Loop-Helix Transcription Factors of the Anthocyanin Pathway. Quattrocchio, F., Verweij, W., Kroon, A., Spelt, C., Mol, J., Koes, R. Plant Cell (2006)