Transgenic plants have become attractive systems for production of human therapeutic proteins because of the reduced risk of mammalian viral contaminants, the ability to do large scale-up at low cost, and the low maintenance requirements. Here we report a feasibility study for production of a human therapeutic protein through transplastomic transformation technology, which has the additional advantage of increased biological containment by apparent elimination of the transmission of transgenes through pollen. We show that chloroplasts can express a secretory protein, human somatotropin, in a soluble, biologically active, disulfide-bonded form. High concentrations of recombinant protein accumulation are observed (>7% total soluble protein), more than 300-fold higher than a similar gene expressed using a nuclear transgenic approach. The plastid-expressed somatotropin is nearly devoid of complex post-translational modifications, effectively increasing the amount of usable recombinant protein. We also describe approaches to obtain a somatotropin with a non-methionine N terminus, similar to the native human protein. The results indicate that chloroplasts are a highly efficient vehicle for the potential production of pharmaceutical proteins in plants.

nodavirus V iruses whose host range spans two kingdoms are few. Most are insect-vectored plant viruses that multiply in a narrow insect host range (1). Flock house virus (FHV), a member of the insect͞animal virus family called Nodaviridae (for reviews, see refs. 2-4), is a unique example of a virus that crosses the kingdom barrier. FHV was originally isolated from the New Zealand grass grub Costelytra zealandica (5). The range of insect hosts for FHV is not well studied. In the laboratory, FHV replicates in the larvae of the wax moth Galleria mellonella and cultured Drosophila melanogaster cells (4). In 1990, FHV was shown to replicate and produce infectious virions in barley protoplasts and the inoculated leaves of several other monocotyledon and dicotyledon plant species without producing symptoms (6). FHV thus replicates in both insects (2) and plants (6), although it is not transmitted to plants by its insect hosts (2).The genome of FHV consists of two small messenger sense RNAs: RNA1 (3.1 kb; ref. Broad host range, simple genome organization, and asymptomatic growth in plants make FHV a powerful tool for the study of fundamental questions of virus-host interactions and the modification of gene expression in plants. A drawback to its use is that FHV does not encode a protein that enables it to move inside the plant. In all plants previously tested, FHV accumulated only in inoculated tissues or cells (6).Here we report the results of experiments designed to determine whether FHV could be mobilized in plants with the use of plant viral movement proteins (MPs). We used genes encoding the 30-kDa protein of tobacco mosaic virus (TMV) and the 35-kDa protein of red clover necrotic mosaic virus (RCNMV) (dianthovirus), two of the most well-studied and widely used plant viral MPs, both expressed as transgenes in N. benthamiana plants (11-16). These plant viruses each encode a single, functionally homologous MP with conserved amino acid sequences (14, 17), and neither virus requires coat protein for cell-to-cell movement (16, 18). These MPs have been shown conclusively to bind RNA in vitro and to increase the size exclusion limit of plasmodesmata in mesophyl cells to allow cell-to-cell movement (19). Moreover, both TMV MP and RCNMV MP have been shown to complement the transport of other plant viruses belonging to different taxonomic groups, such as barley stripe mosaic virus (20, 21), potato virus X (22), and brome mosaic virus (23).Our results show that MPs of both TMV and RCNMV initiate local as well as long-distance movement of FHV in MP transgenic plants. RCNMV MP is more effective in mobilizing movement of FHV to noninoculated leaves. We also show that FHV replicates in inoculated leaves of six more plant species, in addition to those reported previously (6). Newly discovered experimental hosts of FHV include alfalfa, Arabidopsis, Brassica, cucumber, rice, and sweet corn. Plants were grown from seeds planted in Scotts Redi-earth Plug and Seedling Mix and placed in growth chambers maintained at 24°C, with 12 h of l...

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