The transcriptional regulator OPAQUE2 and the nitrogen supply interact in regulating the overall protein synthesis in endosperm
--G. Donini, M. Maddaloni, C. Balconi, E. Rizzi, P. Gallusci*, F. Forlani, L. Stasse and M. Motto

The OPAQUE2 (O2) protein is a bZIP class transcription factor involved in the regulation of endosperm proteins including the 22-kD zein storage proteins. In this study we have focused our attention on the relationship between O2 and genes encoding enzymes presumed to have pivotal roles in nitrogen (N) metabolism. The genes tested were for glutamine synthetase 1-2 (GS1-2), acetolactate synthetase (ALS), and pyruvate ortophosphate dikinase (PPDK) which all proved to be sensitive to the presence of a functional O2 allele. Particular attention has been given to the regulatory relationship between O2 and PPDK because the latter is suspected to be an enzyme that, in its cytosolic form, is responsible for carbon (C) partitioning between starch and proteins. Data were obtained indicating that O2 regulates the cyPPDK1 gene directly and specifically. The results support the hypothesis that O2, besides being an activator of 22 kD zein genes, plays a role in controlling the overall N metabolism, thus mimicking the role that the yeast transcriptional activator GCN4 plays in the general control. Finally, it is reported that the N supply to the endosperm interacts with O2, regulating the transcription of the genes for ALS, GS1-2, PPDK and O2 itself. On the basis of the data obtained, a model is presented in which two levels of control of endospermic protein synthesis are active: a first level of control exerted by O2, while a second level is driven by the overall state of the metabolism, possibly via regulators different from O2. Moreover, the two regulative mechanisms are interconnected in that O2 influences genes which are fluxing C and N compounds while the N supply influences the regulation of O2 transcription.
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