Functional analysis of a light-responsive plant bZIP transcriptional regulator.

Common plant regulatory factor 1 (CPRF1) is a parsley basic region/leucine zipper (bZIP) transcription factor that recognizes specific nucleotide sequences containing ACGT cores. Such a sequence is contained within LRU1, the composite light regulatory unit that is necessary and sufficient for light-dependent activity of the parsley chalcone synthase (CHS) promoter. After light treatment of both etiolated and green seedlings, 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 the CPRF1 promoter fused to the beta-glucuronidase (GUS) open reading frame 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 elements at a distance of 12 bp between their palindromic centers. This feature is reminiscent of as-1 and octopine synthase (ocs) elements identified in promoters from plant pathogens. This double ACGT Element element, designated dACECPRF1, stimulated transcription when placed 5' to a heterologous core promoter. CPRF1 bound to dACECPRF1 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 activities of light-regulated promoters.