Transcriptional and Posttranscriptional Regulation of Nitrogen-Responding Expression of Phosphoenolpyruvate Carboxylase Gene in Maize

To study the regulation of gene expression for enzymes in the C4 photosynthetic pathway of maize (Zea mays L.) in response to changing N status in developing photosynthetic cells, we have studied in vitro transcription of the phosphoenolpyruvate carboxylase (PEPC) gene in leaf nuclei isolated from plants during recovery from N starvation. The induction was specific for the C4-type PEPC gene (C4Ppc1), and its transcription was N dependent and increased markedly by supply of an N source, but there was a discrepancy between the steady-state levels of mRNA and the stimulation of in vitro transcription. The results suggest that the N-inducible expression of C4Ppc1 is regulated both transcriptionally and posttranscriptionally by N availability. The in vitro transcription rate of C4Ppc1 was greatly stimulated by incubating detached leaves with zeatin alone, whereas the rate remained essentially unchanged by incubating with an exogenous N source alone. The results, taken together, imply that cytokinins up-regulate the transcription of C4Ppc1 in response to N status, whereas glutamine and/or its metabolite(s) up-regulate the level of the transcript. The transcription was totally inhibited by cycloheximide, indicating that the cytokinin-dependent transcription of C4Ppc1 requires the synthesis of protein.