Alteration of the specificity of the Streptomyces subtilisin inhibitor by gene engineering.

We have altered the amino acid at the center of the reactive site (methionine 73) of Streptomyces subtilisin inhibitor (SSI) by site-directed and cassette mutagenesis. Replacement by lysine or arginine resulted in trypsin inhibitory activity, replacement only by lysine gave inhibition of lysyl endopeptidase, and replacement by tyrosine or tryptophan resulted in inhibition of alpha-chymotrypsin. The four mutant SSIs retained their native activity against subtilisin BPN'. Thus by altering only one amino acid residue at the reactive site of SSI to the substrate specificity of the respective protease we could successfully change its inhibitory profile.