CPRF4A, A NOVEL PLANT BZIP PROTEIN OF THE CPRF FAMILY - COMPARATIVE ANALYSES OF LIGHT-DEPENDENT EXPRESSION, POSTTRANSCRIPTIONAL REGULATION,NUCLEAR IMPORT AND HETERODIMERIZATION

Several DNA-binding proteins with conserved basic region/leucine zippe r domains (bZIP) have been isolated from parsley. They all recognise d efined ACGT-containing elements (ACEs), including ACE(PcCHSII) in the Light Regulatory Unit LRU1 of the CHS promoter which confers light res ponsiveness. A new member of this Common Plant Regulatory Factor (CPRF ) family, designated CPRF4a, has been cloned, which displays sequence similarity to HBP-1a from wheat, as well as to other plant bZIP protei ns. CPRF4a specifically binds as a homodimerto ACE(PcCHSII) and forms heterodimers with CPRF1 but not with CPRF2. In adult parsley plants, C PRF2 and CPRF4a mRNAs are found in all tissues and organs in which the chalcone synthase gene CHS is expressed. In protoplasts from suspensi on cultured cells, UV irradiation (290-350 nm) did not cause an increa se in levels of CPRF1, CPRF2, or CPRF4a mRNA, whereas the correspondin g CPRF proteins accumulated within 15 min of light treatment. Furtherm ore, the rapid light-mediated increase of CPRF proteins was insensitiv e to transcriptional inhibitors, suggesting that a post-transcriptiona l mechanism controls CPRF accumulation. CPRFs as well as Arabidopsis t haliana G-box binding factors (GBFs) are selectively transported from the cytosol into the nucleus, as shown in an in vitro nuclear transpor t system prepared from evacuolated parsley protoplasts, indicating tha t cytosolic compounds are involved in regulated nuclear targeting of p lant bZIP factors.