Molecular biology of C₄ photosynthesis in Zea mays: differential localization of proteins and mRNAs in the two leaf cell types

We have investigated the molecular basis of differential localization of enzyme activities in mesophyll(M) and bundle-sheath (B) cells of maize leaves. M protoplasts and B strands were prepared by enzymatic digestions and mechanical treatment of secondary leaves. Soluble and thylakoid membrane proteins from the two cell types were compared by one- and two-dimensional gel electrophoresis and quantitative rocket immunoelectrophoresis. In addition, several thylakoid polypeptides were identified by crossed immunoelectrophoresis using monospecific antibodies. M and B thylakoids show quantitative and qualitative differences in their polypeptide compositions. While the M thylakoids contain the normal complement of polypeptides, the B thylakoids are deficient in ferredoxin-NADP⁺ reductase, photosystem II reaction center polypeptides, and the light-harvesting chlorophyll a/b-protein complex. Comparison of the soluble proteins by two-dimensional gel electrophoresis revealed marked differences between M and B cells. The major proteins of one cell type are clearly absent from the other. These differences are paralleled by differences in the in vitro translation products of poly A⁺ RNA isolated from the two cell types. Immunoprecipitation experiments showed that mRNA encoding the small subunit of ribulose-1,5-bisphosphate carboxylase (rbcS) is localized exclusively in B cells, whereas mRNA encoding phosphoenolpyruvate carboxylase is detected only in M cells. cDNA clones encoding the carboxylase rbcS and the chlorophyll a/b binding protein were used as probes in Northern blot analysis. M cells contain no detectable RNA encoding rbcS but have a higher steady state level of RNA encoding the chlorophyll a/b-binding polypeptide compared to B cells. Taken together, our results demonstrate that differential gene expression in the two leaf cell types is regulated at the level of translatable mRNA, and, for at least two proteins, at the level of steady-state RNA.