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We describe a new class of mutants of Escherichia coli K-12 defective in conjugation (Con(-) mutants). They lack a major protein of the outer membrane and are defective as recipients with Hfr and F' donors and as recipients for several F-like R factors and Col factors. In the case of crosses with an Hfr donor, we have shown that the Con(-) recipient is defective in pair formation. The mutants are resistant to certain phages due to loss of receptor activity.
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