Green Factory: Recombinant Protein Production in Chloroplasts

erated by introducing foreign genes into the nuclear genome. Here we focus on plastid transformation as an alternative. Plastids have their own rudimentary genome, the plastome, which encodes around 120 genes [1]. Plastids in green tissues are “chloroplasts”, whose primary function is photosynthesis. In contrast to the nuclear genome with typically two copies of each gene per cell, the plastome is present in over 10,000 copies (a leaf cell has up to 100 chloroplasts each containing about 100 plastome molecules). Due to this high gene dosage the plastid represents a very attractive compartment for high-level expression of recombinant proteins. Site-directed insertion of the transgenes into the plastid avoids ‘position effects’ and leads to uniform and reproducible expression. Silencing of transgenes, also often observed in the nucleus, is not known for plastids. It is possible to co-express several genes by creating artificial operons. Furthermore, since plastid traits in most crop plants are not inherited through the pollen, spread of novel genes to other crops or closely related wild species is prevented [2]. This is a major contribution to bio-safety and should reduce environmental concerns relating to GM plants. Plastid transformation vectors carry a gene of interest together with a selection marker between flanking sequences, which direct site-specific insertion into the plastome (Fig. 2a). DNA can be delivered to the chloroplast via particle bombardment R