The regulation of PEP carboxylase gene expression by nitrogen in maize leaf tissue

通过分析玉米叶片基区（即光合成熟细胞）在n胁迫恢复过程中的代谢中间体、酶活性、蛋白质和mrna，得到了以下结果。(A) PEP羧化酶（PEPC）的选择性积累主要在转录水平上受n有效性的调控。研究了诱导PEPC n依赖性基因表达的n效度。结果表明，谷氨酰胺可能是控制PEPC n依赖基因表达的一个积极信号。谷氨酰胺的积累主要是由于氮胁迫恢复过程中谷氨酰胺合成酶和铁氧化还蛋白依赖性谷氨酸合成酶的差异表达所致。(B)除n源外，细胞分裂素是n胁迫植物离体叶片中PEPC基因诱导的重要成分，具有激素特异性。(C)对比分析了光合作用c4途径核心酶碳酸酐酶（CA）与PEPC在n胁迫恢复过程中的基因表达情况。结果表明，CA和PEPC在响应氮有效性方面受到相同或非常相似的协调调节机制。(D)测定了不同氮条件下叶肉细胞和束鞘细胞胞浆中谷氨酰胺合成酶的基因表达。然而，无论氮条件如何，编码该酶的MRNA水平不变。这表明至少对PEPC和CA基因来说，n依赖的基因表达调控是特异性的。

The following results were obtained by analyzing metabolic intermediates, enzyme activities, proteins, and mRNAs in the basal region of leaves, namely, photosynthetically maturing cells, of maize plants during recovery from N-stress.(A)The selective accumulation of PEP carboxylase(PEPC)is regulated primarily at transcriptional level in responding to N-availability. The N-avaialbility for induction of the N-dependent gene expression of PEPC was studied. The results obtained indicate that glutamine can be a positive signal for the control of the N-dependent gene expression of PEPC. The results also suggested that the accumulation of glutamine is mainly due to the differential expression of glutamine synthetase and ferredoxin-dependent-glutamate synthase during recovery from N-stress.(B)In addition to N-source cytokinin is an essential component for the induction of PEPC gene in the detached leaves of N-stressed plants, showing a hormonal specificity.(C)The gene expression of carbonic anhydrase(CA), a core enzyme in the C4pathway of photosynthesis, was analyzed in comparison to that of PEPC during recovery from N-stress. The results obtained show that CA and PEPC are coordinately regulated by the same or very similar mechanism in responding to N-availability.(D)The gene expression of glutamine synthetase located in the cytosols of mesophyil and bundle sheath cells was examined under various N-conditions. However, the levels of MRNA encoding for this enzyme unchanged regardless of N condition. This indicates that the N-dependent regulation of gene expression is specific at least for PEPC and CA genes.