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Plasmid formation by N derivatives of lambdoid phages has been reinvestigated with transducing phages carrying the trp, lac and gal genes of Escherichia coli. Transduction by λN cI derivatives was inefficient and short-lived in each case, under both selective and nonselective conditions. Mutant operators were introduced to relieve possible auto-repression by the cro gene product. Such N-defective phage genomes were able to propagate continually as plasmids, although without selection they were gradually lost from the carrier cells. Plasmid formation remained inefficient, however. The entire chromosome of N phages can be expressed by transcription that leaks through the serially arranged Rho-dependent terminators. Some functions so expressed are deleterious to the plasmid state and cause the instability of λN plasmids.
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