THE PROMOTERS OF 2 CARBOXYLASES IN A C-4 PLANT (MAIZE) DIRECT CELL-SPECIFIC, LIGHT-REGULATED EXPRESSION IN A C-3 PLANT (RICE)

C-4 plants have two carboxylases which function in photosynthesis. One , phosphoenolpyruvate carboxylase (PEPC) is localized in mesophyll cel ls, and the other, ribulose bisphosphate carboxylase (RuBPC) is found in bundle sheath cells. In contrast, C-3 plants have only one photosyn thetic carboxylase, RuBPC; which is localized in mesophyll cells. The expression of PEPC in C-3 mesophyll cells is quite low relative to PEP C expression in C-4 mesophyll cells. Two chimeric genes have been cons tructed consisting of the structural gene encoding beta-glucuronidase (GUS) controlled by two promoters from C-4 (maize) photosynthetic gene s: (i) the PEPC gene (pepc) and (ii) the small subunit of RuBPC (rbcS) . These constructs were introduced into a C-3 cereal, rice. Both chime ric genes were expressed almost exclusively in mesophyll cells in the leaf blades and leaf sheaths at high levels, and no or very little act ivity was observed in other cells. The expression of both genes was al so regulated by light. These observations indicate that the regulation systems which direct cell-specific and light-inducible expression of pepc and rbcS in C-4 plants are also present in C-3 plants. Neverthele ss, expression of endogenous pepc in C-3 plants is very low in C-3 mes ophyll cells, and the cell specificity of rbcS expression in C-3 plant s differs from that in C-4 plants. Rice nuclear extracts were assayed for DNA-binding protein(s) which interact with a cis-regulatory elemen t in the pepc promoter. Gel-retardation assays indicate that a nuclear protein with similar DNA-binding specificity to a maize nuclear prote in is present in rice. The possibility that differences in pepc expres sion in a C-3 plant (rice) and C-4 plant (maize) may be the result of changes in cis-acting elements between pepc in rice and maize is discu ssed. it also appears that differences in the cellular localization of rbcS expression are probably due to changes in a transacting factor(s ) required for rbcS expression.