Chidi Afamefule and Christine Raines

C4 photosynthesis is characterized by the compartmentalization of the processes of atmospheric uptake of CO2 and its conversion into carbohydrate between mesophyll and bundle-sheath cells. As a result, most of the enzymes participating in the Calvin-Benson-Bassham (CBB) cycle, including RubisCO, are highly expressed in bundle-sheath cells. There is evidence that changes in the regulatory sequences of RubisCO contribute to its bundle-sheath-specific expression. However, little is known about how the expression pattern of other CBB cycle enzymes is regulated. In this study, we use a computational approach to scan for transcription factor binding sites in the regulatory regions of the CBB enzymes, SBPase, FBPase, PRK, and GAPDH-B, of C3 and C4 grasses. We identified putative trans-acting regulators of the expression of CBB cycle enzymes, with binding site motifs present in each enzyme regardless of the photosynthetic path used by the plant. In addition, we also found C3- and C4-specific transcription factor binding sites that could contribute to the pathway specific regulation of gene expression. These results provide a foundation for the functional analysis of the differences in regulation of CBB cycle enzymes between C3 and C4 grasses.