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C(4) photosynthesis depends on the strict compartmentalization of CO(2) assimilatory enzymes. cis-regulatory mechanisms are described that ensure mesophyll-specific expression of the gene encoding the C(4) isoform of phosphoenolpyruvate carboxylase (ppcA1) of the C(4) dicot Flaveria trinervia. To elucidate and understand the anatomy of the C(4) ppcA1 promoter, detailed promoter/reporter gene studies were performed in the closely related C(4) species F. bidentis, revealing that the C(4) 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 beta-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 C(3) 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.
Gowik et al. (Sat,) studied this question.