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Protein 1, a major protein of the outer membrane ofEscherichia coli , has been shown to be the pore allowing the passage of small hydrophilic solutes across the outer membrane. InE. coli K-12 protein 1 consists of two subspecies, 1a and 1b, whereas inE. coli B it consists of a single species which has an electrophoretic mobility similar to that of 1a. K-12 strains mutant at theompB locus lack both proteins 1a and 1b and exhibit multiple transport defects, resistance to toxic metal ions, and tolerance to a number of colicins. Mutation at thetolF locus results in the loss of 1a, in less severe transport defects, and more limited colicin tolerance. Mutation at thepar locus causes the loss of protein 1b, but no transport defects or colicin tolerance. Lysogeny ofE. coli by phage PA-2 results in the production of a new major protein, protein 2. Lysogeny of K-12ompB mutants resulted in dramatic reversal of the transport defects and restoration of the sensitivity to colicins E2 and E3 but not to other colicins. This was shown to be due to the production of protein 2, since lysogeny by phage mutants lacking the ability to elicit protein 2 production did not show this effect. Thus, protein 2 can function as an effective pore.ompB mutations inE. coli B also resulted in loss of protein 1 and similar multiple transport defects, but these were only partially reversed by phage lysogeny and the resulting production of protein 2. When theompB region fromE. coli B was moved by transduction into anE. coli K-12 background, only small amounts of proteins 1a and 1b were found in the outer membrane. These results indicate that genes governing the synthesis of outer membrane proteins may not function interchangeably between K-12 and B strains, indicating differences in regulation or biosynthesis of these proteins between these strains.

Outer Membrane Proteins of Escherichia coli VII. Evidence That Bacteriophage-Directed Protein 2 Functions as a Pore