Density alterations associated with transducing ability in the bacteriophage lambda

The transducing variant of the bacteriophage λ is thought to arise by a double crossover in which a section of the normal chromosome is exchanged for a region of the bacterial chromosome containing galactose fermentation markers. If the exchange were unequal, the resulting transducing phages should have an altered DNA content which might be detected as a modification of the density of the phage particle. An examination of ten independently arising populations of transducing phages showed that each had a different but essentially uniform density. In contrast, the non transducing phage contained in these lysates possessed a constant density. The density of a transducing phage was stable to a cycle of lysogenization, prophage replication and multiplication following induction, as well as to genetic interactions with normal A and with the bacterial chromosome. Assuming the observed density changes to be due to changes in DNA content, it was calculated that the maximum change corresponded to 10 7 mol wt units, or 1·5 × 10 4 nucleotide pairs, and the smallest to 800 nucleotide pairs. It is proposed that the unequal exchanges giving rise to the density alterations are the result of poor homology and incomplete pairing between the Gal region of the bacterial chromosome and the region of λ which is deleted.