FUNCTIONAL-ANALYSIS OF A LIGHT-RESPONSIVE PLANT BZIP TRANSCRIPTIONAL REGULATOR

Common plant regulatory factor 1 (CPRF1) is a parsley basic region/leu cine zipper (bZIP) transcription factor that recognizes specific nucle otide sequences containing ACGT cores. Such a sequence is contained wi thin LRU1, the composite light regulatory unit that is necessary and s ufficient for light-dependent activity of the parsley chalcone synthas e (CHS) promoter. Af ter light treatment of both etiolated and green s eedlings, CPRF1 mRNA levels increased prior to CHS mRNA accumulation. The change in CPRF1 mRNA leads to a light-responsive increase in CPRF1 protein. Transient expression analysis in parsley protoplasts using t he CPRF1 promoter fused to the beta-glucuronidase (GUS) open reading f rame indicated that light-dependent CPRF1 mRNA accumulation was under transcriptional control. The 5' untranslated region of the CPRF1 gene includes a cis-acting nucleotide sequence that contains two ACGT eleme nts at a distance of 12 bp between their palindromic centers. This fea ture is reminiscent of as-1 and octopine synthase (ocs) elements ident ified in promoters from plant pathogens. This double ACGT Element elem ent, designated dACE(CPRF1), stimulated transcription when placed 5' t o a heterologous core promoter. CPRF1 bound to dACE(CPRF1) DNA as well as to the ACGT element from the CHS promoter in vitro. Cotransfection experiments demonstrated that CPRF1 interacts with these elements in vivo and that overexpression of CPRF1 actually reduced light-dependent transcription from the CHS promoter. CPRF1 thus appears to contribute to the regulation of the CPRF1 gene and to interfere with the activit ies of light-regulated promoters.