Sheen J Y, Bogorad L
J Biol Chem. 1987 Aug 25;262(24):11726-30.
Pyruvate orthophosphate dikinase, phosphoenolpyruvate carboxylase, and NADP-malate dehydrogenase function in a series of reactions for fixing CO2 in mesophyll cells and NADP-malic enzyme (ME) catalyzes the production of CO2 and NADPH in bundle sheath cells of maize which is a NADP-ME type C4 plant. Northern blot analyses with cDNA clones for pyruvate orthophosphate dikinase and phosphoenolpyruvate carboxylase and in vitro translation-immunoprecipitation experiments with antiserum to NADP-malate dehydrogenase showed that pools of transcripts of these three genes grow and shrink coordinately in mesophyll cells but not in bundle sheath cells upon illumination of dark-grown maize seedlings. Western blot analyses indicated that the protein levels of phosphoenolpyruvate carboxylase and pyruvate orthophosphate dikinase are low in dark-grown maize seedlings and increase progressively following light-induced transient accumulation of their mRNAs in mesophyll cells. These proteins continue to accumulate and plateau in late-greening and green leaves in spite of a rapid drop in the sizes of their mRNA pools. Surprisingly, relatively large amounts of NADP-malate dehydrogenase are present in mesophyll cells of etiolated leaves despite the low level of the corresponding mRNA. No phosphoenolpyruvate carboxylase or NADP-malate dehydrogenase were detected in bundle sheath cells. On the other hand, the ME gene responds to light induction at both the transcriptional and translational levels only in bundle sheath cells. Moreover, the steady-state level of ME mRNA stays high in late-greening and green leaves in contrast to the rapid decline of mRNA levels of three other C4 pathway genes in mesophyll cells. In addition, low levels of both the mRNA and protein encoded by the PPDK gene were detected in bundle sheath cells. These levels were not influenced by light as distinguished from the patterns observed in mesophyll cells.
丙酮酸磷酸双激酶、磷酸烯醇式丙酮酸羧化酶和NADP - 苹果酸脱氢酶在叶肉细胞中参与一系列固定二氧化碳的反应，而NADP - 苹果酸酶（ME）在玉米的维管束鞘细胞中催化二氧化碳和NADPH的生成，玉米是一种NADP - ME型C4植物。用丙酮酸磷酸双激酶和磷酸烯醇式丙酮酸羧化酶的cDNA克隆进行Northern杂交分析，并用抗NADP - 苹果酸脱氢酶的抗血清进行体外翻译 - 免疫沉淀实验，结果表明，在黑暗中生长的玉米幼苗受光照后，这三个基因的转录本库在叶肉细胞中协同增长和收缩，但在维管束鞘细胞中并非如此。Western杂交分析表明，在黑暗中生长的玉米幼苗中，磷酸烯醇式丙酮酸羧化酶和丙酮酸磷酸双激酶的蛋白水平较低，随着它们的mRNA在叶肉细胞中光诱导的瞬时积累，其蛋白水平逐渐增加。尽管它们的mRNA库大小迅速下降，但这些蛋白在叶片晚绿期和绿叶期继续积累并趋于平稳。令人惊讶的是，尽管相应mRNA水平较低，但黄化叶片的叶肉细胞中仍存在相对大量的NADP - 苹果酸脱氢酶。在维管束鞘细胞中未检测到磷酸烯醇式丙酮酸羧化酶或NADP - 苹果酸脱氢酶。另一方面，ME基因仅在维管束鞘细胞的转录和翻译水平上对光诱导作出反应。此外，与叶肉细胞中其他三个C4途径基因的mRNA水平迅速下降形成对比的是，ME mRNA的稳态水平在叶片晚绿期和绿叶期保持较高。此外，在维管束鞘细胞中检测到PPDK基因编码的mRNA和蛋白水平较低。与在叶肉细胞中观察到的模式不同，这些水平不受光照影响。
