Homologous genes for the C4 isoform of phosphoenolpyruvate carboxylase in a C3 and a C4 Flaveria species.

The C4 isoform of phosphoenolpyruvate carboxylase (PEPCase) in Flaveria trinervia is encoded by the ppcA subgroup of the PEPCase gene family and is abundantly expressed in the mesophyll cells of leaves. The homologous ppcA genes in the C3 plant F pringlei are only weakly expressed and their transcripts do not show the strictly leaf-specific accumulation pattern observed for the F. trinervia genes. Two representative members of the ppcA subfamilies of F. trinervia (C4) and F. pringeli (C3)-named ppcA1-were characterized by Southern blotting, nucleotide sequencing and primer extension analysis. Comparison of the deduced amino acid sequences reveals a close similarity between C4 and C3 isoforms. Only few C4-specific positions can be detected when all known plant PEPCases are included in the comparison. A regulatory domain involved in light-dependent phosphorylation/dephosphorylation of the C4 and crassulacean acid metabolism (CAM) isoforms is present in the ppcA1 gene products of both the C3 and C4 Flaveria. The 5' flanking regions are essentially homologous. The putative promoter regions share several identical sequence motifs (CCAAT, AT-1 and GT-1 box III/III* elements). Additionally, alterations in elements that could contribute to differences in expression rates and light regulation are found. The significance of these findings is discussed with respect to the molecular evolution of C4 photosynthesis in Flaveria.