cis-Regulatory Elements for Mesophyll-Specific Gene Expression in the C4 Plant Flaveria trinervia, the Promoter of the C4 Phosphoenolpyruvate Carboxylase Gene

C4 photosynthesis depends on the strict compartmentalization of CO2 assimilatory enzymes. cis-regulatory mechanisms are described that ensure mesophyll-specific expression of the gene encoding the C4 isoform of phosphoenolpyruvate carboxylase (ppcA1) of the C4 dicot Flaveria trinervia. To elucidate and understand the anatomy of the C4 ppcA1 promoter, detailed promoter/reporter gene studies were performed in the closely related C4 species F. bidentis, revealing that the C4 promoter contains two regions, a proximal segment up to −570 and a distal part from −1566 to −2141, which are necessary but also sufficient for high mesophyll-specific expression of the β-glucuronidase reporter gene. The distal region behaves as an enhancer-like expression module that can direct mesophyll-specific expression when inserted into the ppcA1 promoter of the C3 plant F. pringlei. Mesophyll expression determinants were restricted to a 41-bp segment, referred to as mesophyll expression module 1 (Mem1). Evolutionary and functional studies identified the tetranucleotide sequence CACT as a key component of Mem1.