Maize rbcS promoter activity depends on sequence elements not found in dicot rbcS promoters.

Although the molecular mechanisms of dicot photosynthetic gene regulation have been pursued actively, comparable studies of monocot regulation have been slow to come forth. We show here that monocot (maize and wheat) but not dicot (pea, tobacco, and Arabidopsis) ribulose-1,5-bisphosphate carboxylase small subunit (rbcS) gene promoters are active in maize mesophyll protoplasts. The evolutionarily conserved GT and G boxes of dicot rbcS promoters are not essential for light-responsive expression in monocot leaf cells. Instead, at least six constitutive and light-sensitive regulatory elements are likely important for maize rbcS expression. Synergism between upstream and downstream promoter elements is required. Whereas in dicots, light triggers coupled leaf development and photosynthetic gene expression, in monocots, light regulation of rbcS is uncoupled from leaf development. Light regulation of maize rbcS may be divided into direct and indirect contributions mediated by different regulatory elements. Because wheat and maize rbcS promoters show sequence homologies and similar expression patterns in monocot and dicot leaf cells, it appears likely that monocots share conserved regulatory elements irrespective of whether they utilize the C3 or C4 pathway for carbon fixation.