A plastid transformation and expression vector which comprises an expression cassette comprising as operably linked components, in the 5' to the 3' direction of translation, a promoter operative in said plastid, a selectable marker sequence, a heterologous DNA sequence coding for cholera toxin B-subunit fused to proinsulin, a transcription termination region functional in said plastid, and flanking, each side of the expression cassette, flanking DNA sequences which are homologous to a DNA sequence inclusive of a spacer sequence of the target plastid genome, and, whereby stable integration of the heterologous coding sequence into the plastid genome of the target plant is facilitated throughout homologous recombination of the flanking sequence with the homologous sequences in the target plastid genome.
