Synthesis and protein body deposition of maize 15-kd zein in transgenic tobacco seeds.

The maize 15-Kd zein structural gene was placed under the regulation of French bean beta-phaseolin gene flanking regions. Agrobacterium tumefaciens-mediated transformation was used to insert the chimeric phaseolin-zein gene into the tobacco genome. Transgenic plants synthesized zein in a tissue-specific manner during the latter half of seed development. Transcription of the chimeric gene was initiated in phaseolin-derived sequences, and was terminated within the phaseolin gene 3' flanking region. Both zein- and phaseolin-derived polyadenylation signals were used in the processing of zein RNA in transgenic plant seeds. Zein accumulation, though subject to an 80-fold variation among 19 plants tested, could reach as much as 1.6% of the total seed protein in several plants. In developing tobacco seeds, zein was correctly processed by the removal of a 20-amino-acid signal peptide. Electron microscope immunogold localization of the zein expressed in embryo and endosperm tissue indicates that the monocot protein accumulates in the crystalloid component of vacuolar protein bodies. The density of gold label over the protein bodies is several fold greater in the embryo than the endosperm. Zein is found in roots, hypocotyls and cotyledons of germinating transgenic tobacco seeds.