Dof1 and Dof2 transcription factors are associated with expression of multiple genes involved in carbon metabolism in maize

Dof proteins are transcription factors that appear to be unique to plants. Maize Dof1 has been suggested to be a regulator for C4 photosynthetic phosphoenolpyruvate carboxylase (C4PEPC) gene expression. The present study demonstrates that Dof1 also enhances transcription from the promoters of both cytosolic orthophosphate dikinase (cyPPDK) genes and a non-photosynthetic PEPC gene, which are not present in animals. Expression of Dof1-specific antisense RNA or the DNA-binding domain of Dof1 alone reduced the activities of these promoters in maize leaf protoplasts. Electrophoretic mobility shift assays revealed several Dof1-binding sites in these promoters. The cyppdk1 promoter contained two Dof1-binding sites, one of which was linked to the binding site of a plant bZIP protein. By using deleted or mutated cyppdk1 promoters, both Dof1-binding sites were shown to be functional. Furthermore, Dof1 elevated the activities of the cyppdk and pepc promoters more strongly in greening protoplasts than in etiolated protoplasts, in accordance with the different activities of these promoters in two types of protoplasts. Another Dof protein of maize, Dof2, suppressed the activity of the C4pepc promoter but was able to activate certain other promoters. These results suggest that Dof proteins may play regulatory roles in multiple gene expressions associated with the plant-specific pathway for carbon metabolism in maize. In addition, the primary characteristic of Dof proteins, i.e. different activities in distinct types of cells and opposite actions on promoters in different contexts, suggests the potential of Dof proteins to differentially regulate diverse promoters in a variety of plant tissues. Speculation raised by these results concerning the evolution of the C4pepc gene is also discussed.