Diverse mechanisms regulate the expression of genes coding for C-4 enzymes

Most of the enzymes of the C-4 pathway of photosynthesis are compartmentalised in either mesophyll or bundle sheath cells. We have begun to dissect the mechanisms controlling the cell-specific expression of genes coding for C-4 enzymes by locating the regulatory DNA sequences from two genes in the transformable C-4 dicot, Flaveria bidentis. We show that chloroplast and cytosolic forms of pyruvate,orthophosphate dikinase are encoded by a single gene, Pdk. By fusing selected regions of the Pdk gene, to the gusA reporter gene we were able to infer promoter activities from measurements of GUS enzyme activity in transgenic F. bidentis plants. We show that the 5' end of the Pdk gene contains sequences controlling high-level, mesophyll-specific expression of the C-4 form of the enzyme. The promoter controlling low-level expression of the cytosolic form was located in the large intron. The C-4 isoform of NADP-malic enzyme is encoded by the Me1 gene. Analyses of a series of reporter gene fusions showed that sequences at the 5' end of the gene control bundle sheath expression but that sequences located near the 3' end are necessary for high-level expression. The mechanisms regulating the expression of both genes are clearly different and suggest that there is no universal mechanism controlling the expression of genes coding for C-4 enzymes.