Expression of the pea albumin 1 gene in transgenic white clover and tobacco.

In order to improve the quality of pasture protein for ruminant animal nutrition, we are introducing genes encoding rumen-protected proteins, rich in essential amino acids, into white clover (Trifolium repens L.). We have introduced a chimaeric gene transcribed from the 35S CaMV promoter, and encoding the pea albumin 1 (PA1) protein, rich in sulphur amino acids, into the white clover genotype WR8 by Agrobacterium-mediated transformation. A transgenic plant with high levels of PA1 mRNA was crossed with a commercial genotype from cv. Regal Ladino and both the parent and progeny plants were analyzed for expression and accumulation of PA1 gene products. Steady-state mRNA levels and transcript sizes in transgenic parent and progeny were comparable. The abundance and stability of the PA1 protein in transgenic white clover plants was examined by immunoselection of in vivo [35S]Na2SO4-labelled plant proteins. Evidence is presented here, that the 11 kDa PA1 proprotein precursor is processed correctly in petiole tissues of newly regenerated white clover plantlets but only the 6 kDa PA1a subunit accumulates in leaflets of tissue-culture-grown and older glasshouse-grown clover plants. Attempts to enhance PA1 abundance by altering its subcellular target in transgenic tobacco plants suggest that the endomembrane system is a relatively stable environment compared with the cytoplasm or chloroplast, for the accumulation of PA1, despite its low abundance there (< 0.001% total cell protein).