Transactivation properties of parsley proline-rich bZIP transcription factors

Light-responsive chalcone synthase (CHS) gene activation requires LRUCHS, a light regulatory promoter unit including the MYB recognition element MRECHS and the ACGT-containing element ACE(CHS). ACECHS is bound by the parsley basic region/leucine zipper (bZIP) factors CPRF1 and 4. Factors containing the bZIP domain exist in animals, plants and yeast, and recognize DNA sequence-specifically after formation of homo- or heterodimers. To determine the potential role of CPRFs in the regulation of CHS promoter activity, we investigated the functions of distinct CPRF domains in a homologous co-transfection system. The proline rich domains of CPRF1 and CPRF4 activate transcription, indicating that CPRF1 and CPRF4 have transactivating properties. Over-expression of the CPRF1 bZIP domain caused a reduction of LRUCHS-mediated light inducibility, and point mutations throughout ACE(CHS) affected both responsiveness to UV-containing white light and transactivation by CPRF1::VP16. The data suggest that a CPRF1-containing bZIP heterodimer interacts with ACE(CHS) in vivo. We discuss regulatory steps in light-induced CHS transcription that may be influenced by CPRF1 and/or related bZIP factors.