Maize C4 photosynthesis involves differential regulation of phosphoenolpyruvate carboxylase genes

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Abstract
Maize as a C4 plant partitions CO2 fixation in two consecutive, spatially separated steps, thus eliminating photorespiration. The crucial enzyme for primary CO2 fixation is a C4-specific phosphoenolpyruvate carboxylase (PEPC). The differential expression of the unique C4-specific gene pepcZm1 and two non-C4-specific genes, pepcZm2A and pepcZm3B, in leaf, root, and stem is reported here. It is shown, in a transient homologous system, that this tissue-specific regulation is mainly controlled by their distinct promoters. The light induction of the C4-specific pepcZm1 in illuminated etiolated (greening) leaves probably relies on light-dependent developmental changes instead of an immediate responsiveness found for other maize genes. Analyses of deleted, mutated, and hybrid promoters revealed the redundant nature of a 14mer which is repeated four times and a decisive function of the TATA box-like motif, TATTT, and the sequences directly preceding it. No consensus sequences to other photosynthetic gene promoters were uncovered. Although light induces the expression of C4 PEPC and other photosynthetic genes in maize, this co-ordination is apparently mediated through different signal transduction pathways and distinct regulatory elements. This study indicates that the acquisition of a new promoter is at least partially responsible for the C4-specific expression of pepcZm1 essential for C4 photosynthesis.