A MEM1-like motif directs mesophyll cell-specific expression of the gene encoding the C₄ carbonic anhydrase in Flaveria

The first two reactions of C₄ photosynthesis are catalysed by carbonic anhydrase (CA) and phosphoenolpyruvate carboxylase (PEPC) in the leaf mesophyll (M) cell cytosol. Translatome experiments using a tagged ribosomal protein expressed under the control of M and bundle-sheath (BS) cell-specific promoters showed transcripts encoding CA3 from the C₄ species Flaveria bidentis were highly enriched in polysomes from M cells relative to those of the BS. Localisation experiments employing a CA3-green fluorescent protein fusion protein showed F. bidentis CA3 is a cytosolic enzyme. A motif showing high sequence homology to that of the Flaveria M expression module 1 (MEM1) element was identified approximately 2 kb upstream of the F. bidentis and F. trinervia ca3 translation start sites. MEM1 is located in the promoter of C₄ Flaveria ppcA genes, which encode the C₄-associated PEPC, and is necessary for M-specific expression. No MEM1-like sequence was found in the 4 kb upstream of the C₃ species F. pringlei ca3 translation start site. Promoter-reporter fusion experiments demonstrated the region containing the ca3 MEM1-like element also directs M-specific expression. These results support the idea that a common regulatory switch drives the expression of the C₄ Flaveria ca3 and ppcA1 genes specifically in M cells.