Complete structure of the gene for phosphoenolpyruvate carboxylase from maize

Phosphoenolpyruvate carboxylase is a key enzyme in photosynthesis in some plants that exploit the C4 photosynthetic pathway for the fixation of CO2. We cloned the gene for this enzyme from maize genomic libraries and analyzed its complete primary structure. The sequence of the cloned gene spans 6781 bp and consists of 10 exons and 9 introns. The site of initiation of transcription is located 84 nucleotides upstream from the first nucleotide of the initiation codon (position -84), as determined by the method of primer-extension analysis. The analysis suggests that there is another initiation site located at position -81. The 5'-flanking region of the gene lacks typical TATA and CCAAT elements in the anticipated regions, but there is a TATA-similar sequence (TATTT) around the -30 regions as well as sequence homologous to the Sp-1 protein-binding site (CCGCCC). Six long, direct repeated sequences and a light-responsive element are also present in the 5'-flanking region. The results of Southern blot analysis indicated that the phosphoenolpyruvate carboxylase gene exists as a small multi-gene family, but the enzyme that is expressed at high levels in green leaves and is involved in C4 photosynthesis is encoded by a single-copy gene in the maize genome.