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 MRECH S 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 seque nce-specifically after formation of homo- or heterodimers. To determine the potential role of CPRFs in the regulation of CHS promoter activity, we inv estigated the functions of distinct CPRF domains in a homologous co-transfe ction system. The proline rich domains of CPRF1 and CPRF4 activate transcri ption, indicating that CPRF1 and CPRF4 have transactivating properties. Ove r-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::V P16. The data suggest that a CPRF1-containing bZIP heterodimer interacts wi th ACE(CHS) in vivo. We discuss regulatory steps in light-induced CHS trans cription that may be influenced by CPRF1 and/or related bZIP factors.