Disclosed herein is an expression cassette containing a polynucleotide encoding an IFN.alpha.2b polypeptide and having two overlapping primers at a 5' end encoding a polyhistidine tag and a thrombin cleavage site fused to the polypeptide, the expression cassette carried by a vector competent for integrating the expression cassette in a plastid genome. Also disclosed is a transgenic plastid, preferably a chloroplast, containing a genome transformed by integration of an expression cassette having a non-plant gene encoding an IFN.alpha.2b polypeptide and having regions that encode a polyhistidine tag and a thrombin cleavage site fused with the IFN.alpha.2b polypeptide.