Abstract: E. coli deficient in protease activities, said activities being not inhibited with diisopropyl fluorophosphate, phenylmethylsulfonyl fluoride, N-.alpha.-tosyl-L-lysyl chloromethyl ketone, N-tosyl-L-phenylalanine chloromethyl ketone, ethylenediamine tetra-acetic acid, leupeptin, antipain, .alpha..sub.2 -macroglobulin or chymostatin, and being inhibited with zinc chloride or copper chloride is disclosed.This E. coli is useful as a host for creating a transformant capable of expressing an exogenous protein or polypeptide.The protein or polypeptide expressed by the transformant may be extracted and purified with a quite low level of decomposition.

Abstract: A method of purifying a polypeptide having a physiological activity such as one having interferon activities from a culture mixture of a microorganism obtained by a recombinant DNA technique and capable of producing the polypeptide is disclosed. The method comprises subjecting the cultured cells to extraction and purification in the presence of a salt of zinc or copper and polyethyleneimine thereby inhibiting decomposition and denaturation of the polypeptide. The extracted polypeptide can be further purified by column chromatographies using a column containing an anion exchange resin, column containing a cation exchange resin and column containing a gel filtration resin.