MOLECULAR CHARACTERIZATION OF THE SPINACH G-BOX BINDING-PROTEIN FAMILY

The promoters of the spinach Ferredoxin-NADP(+)-oxidoreductase gene an d one member of the gene family for the small subunit of ribulose-1,5- bisphosphate carboxylase contain ACGT sequences relevant for gene expr ession. Site-directed mutagenesis revealed that these sequences operat e quantitatively and are nor involved in the light response. We have i solated a cDNA for a basic leucine zipper protein (bZIP) from spinach. After transcription and translation in cell-free systems, the protein binds in vitro to double-stranded oligonucleotides designed according to both sequences, although with different efficiencies. The genomic DNA segment for this bZIP contains 10 introns. The bZIP gene promoter harbors also an ACGT sequence; however. promoter/uidA gene fusions rev ealed that these nucleotides are not essential for expression. At leas t three other genes with high similarities are present in the spinach genome. however, they appear to be either pseudogenes, because they co ntain in-frame stop codons in highly conserved epitopes, or must be po sttranscriptionally modified in order to code for functional proteins. The high sequence similarities suggest that all four sequences derive from gene duplications.