Coexpression of the soybean vegetative storage protein beta subunit (S-VSPbeta) either with the bacterial feedback-insensitive dihydrodipicolinate synthase or with S-VSPalpha stabilizes the S-VSPbeta transgene protein and enhances lysine production in transgenic tobacco plants.

Soybean vegetative storage proteins (S-VSPs) are lysine-rich leaf proteins, originally found to accumulate to high levels in depodded soybean plants. In the present study, we overexpressed S-VSPbeta, the ruminant stable subunit of the S-VSP genes, in transgenic tobacco plants. The S-VSPbeta protein accumulated in all organs studied, but its level declined drastically with leaf age. This instability of S-VSPbeta could be overcome either by elevating free lysine levels or by coexpressing S-VSPbeta with S-VSPbeta. High levels of rumen-stable, lysine-rich proteins is expected to improve absorption of lysine by ruminants. Furthermore, the expression of S-VSPs in heterologous plants led to a significant increase in total soluble lysine, suggesting that these proteins may also permit better assimilation of lysine by humans and monogastric animals.