Monocot Regulatory Protein Opaque-2 1s Localized in the Nucleus of Maize Endosperm and Transformed Tobacco Plants

Protein targeting to the nucleus has been studied extensively in animal and yeast systems; however, nothing is known about nuclear targeting in plants. The Opaque-2 (02) gene produces a regulatory protein that is responsible for inducing transcription of the a-zein class of storage proteins in maize kernels. The cloned 02 gene encodes a protein that contains a leucine zipper DNA binding domain that can interact with zein gene promoters. We have used immunolocalization to show that the 0 2 protein is present in nuclei in the maize endosperm tissues known to produce a-zeins. In addition, neither ambryo tissue from wild-type kernels nor endosperm from kernels harboring a null 02 allele contain the 0 2 protein. Analysis of a transposable, element-induced 02 allele, 02m20, revealed that sectors of endosperm cells contained the nuclear-localized 02 protein, indicating excision of the transposable element. To study further the nuclear transport of the 02 protein, we have transformed this gene, under the control of a constitutive promoter, into tobacco. Plants were shown to have detectable levels of steady-state 02 mRNA and 02 protein. lmmunolocalization of 0 2 protein in transformed tobacco plants indicated that the 02 protein was transported into tobacco nuclei. Therefore, we have developed a system to study nuclear targeting in plants and have established that the nuclear transport machinery is similar in monocots and dicots.