Bacteria mature preproinsulin to proinsulin.

By inserting the rat preproinsulin gene into the bacterial prepenicillinase gene, we formed a variety of hybrid bacterial-eukaryotic signal sequences attached to proinsulin. Among these were the four following constructions: rat proinsulin attached to the entire penicillinase signal sequence and rat preproinsulin fused to all of, to half of, or only to the first four amino acids of the bacterial signal sequence. In all four cases, more than 90% of the rat insulin antigen appeared in the periplasmic space. By immunoprecipitation and determination of the amino acid sequences of the radiolabeled products, we show that the bacteria correctly process both the bacterial and the eukaryotic signal sequences of these hybrid proteins. The cleavage of the eukaryotic signal by bacterial peptidase, in this case, generates proinsulin.