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Published on September 7, 2007; 10.1104/pp.107.103606

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Received June 18, 2007
Accepted September 4, 2007

Wild-type Opaque2 and Defective o2 Polypeptides Form Complexes in Maize Endosperm Cells and Bind the O2-Zein Target Site

Floriana Gavazzi , Barbara Lazzari , Pietro Ciceri , Elisabetta Gianazza , and Angelo Viotti *

Istituto Biologia e Biotecnologia Agraria, Consiglio Nazionale delle Ricerche, Via E. Bassini 15, I-20133 Milano, Italy

* Corresponding author; email: viotti{at}ibba.cnr.it.

The Opaque2 bZIP transcriptional activator controls the expression of several genes in maize. We investigated the phosphorylation extent of wild-type Opaque2 and mutant-defective or mutant-truncated o2 polypeptides in endosperm cells, their subcellular localization, participation in complex formation, and involvement in functional activity. Beside the wild-type, four mutant alleles (o2T, o2-52, o2It, o2-676) producing o2 polypeptides and a null transcript allele (o2R) were considered. Observing the effects of these mutations, multi-phosphorylation events in O2 or o2 proteins were confirmed and further investigated, and the involvement of both the NLS-B and leucine-zipper domains in proper targeting to the nucleus was ascertained. The absence of these domains in the o2T, o2It-S mutant-truncated forms holds them within the cytoplasm, where they are partially phosphorylated, while the presence of NLS-B and a partial leucine-zipper domain in o2-52 distributes this mutant-truncated form in both cytoplasm and nucleus. Although mutated in the NLS-B domain, the o2It-L and o2-676 mutant-defective forms are, respectively, partially or completely distributed into the nucleus. Only wild-type O2 and mutant-defective o2 polypeptides bearing the leucine-zipper are able to form complexes whose components were proven to bind the O2-zein target site by in vitro analyses. The transcription of a subset of H-zein genes as well as H-zein polypeptide accumulation in several o2-mutant-defective genotypes indicate the in vivo involvement of o2-mutant-defective proteins in O2-zein target site recognition. The gathered information broadens our knowledge on Opaque2 functional activity and our view on possible QPM trait manipulation or plant transformation via the utilization of cisgenic elements.






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