An OmpA-deficient mutant of an OmpF/OmpC-free Escherichia coli B-e strain was selected using phage K3. The mutant strain was characterized by SDS-gel electrophoresis, immunoblotting, and electron microscopy. All major outer membrane proteins, including OmpA, were absent. This strain was then transformed with the plasmid pMY222 encoding the K12 OmpF porin or with pBluescript-derived plasmids, encoding the porins OmpC, PhoE, and maltoporin, respectively. Following SDS extraction of outer membrane sacculi from strains expressing individual porins, crystalline porin arrays that allowed in situ structural analysis to be performed were observed. Furthermore, the absence of endogenous major outer membrane proteins facilitated the purification of native porin-lipopolysaccharide complexes, the functionally active channels, from the sacculi of transformed strains. (C) 1993 Academic Press, Inc.