Abstract
From a culture of the lysogenic gal + strain C600 (λ) substrains can be isolated which liberate more gal +-transducing particles per cell than the parent culture. During growth these substrains revert rapidly to the original condition. The transducing phage particles derived from a single substrain are not homogeneous in density. This indicates that each substrain does not carry a defective phage of predetermined density, as is true in the case of a heterogenote strain carrying a λdg prophage. Equilibrium centrifugation analyses of the density distribution of LFT lysates reveal two classes of LFT particles: one class of homogeneous density, like that of active phage λb1 +, and one of particles less dense than λ, which vary widely in density. This suggests the possibility that there are two mechanisms of determination of the final density. Differences are observed in the transducing properties of LFT particles as compared with HFT particles. The LFT particles have an absolute requirement for the presence of an active phage genome in the recipient cell in order to carry out transduction, whereas the HFT particles can transduce at a low rate without active phage. If LFT lysates of one immunity type are used to transduce gal − cells that carry active phage of another immunity type, the resulting clones segregate out transduced cells in which the immunity character of the defective prophage has been changed to that of the prophage carried by the recipient cell. They also segregate out gal + cells which no longer carry any defective phage. Both of these types are exceptional among segregants of gal − cells transduced by HFT lysates. Because of the presence of a density marker b1 closely linked to or included in the c region of one of the two phage strains studied, it was possible to test for the presence or absence of this density marker in the LFT transducing particles that comprise the peak of higher density. This marker appears to be missing in these LFT particles, suggesting a defect in the c region. The b1 marker reappears in the HFT particles, presumably because these are formed by recombination of an LFT particle with an active phage.
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