Formation of inclusion bodies may be the key factor for the stability of expressed products in E. coli.

Des-B30 single-chain insulin gene was constructed from chemically synthesized DNA encoding human proinsulin by oligo-nucleotide induced deletion, cloned into the expression plasmid pBV220, and expressed in E. coli with a level of around 5%. The expressed product was in the form of inclusion bodies. After downstream processing, 45 mg of des-B30 single-chain insulin with a purity of up to 90% was obtained from 1 liter of high density fermentation medium. It is suggested that the fusion protein model to increase the stability of small proteins seems inadequate to some extent, the formation of inclusion bodies may be the key factor for the stability of some expressed products, large or small, in the cytoplasm of E. coli cells.