Gal transduction by phage λ: On the origin and nature of LFT transducing genomes

Summary

There are at least two classes of transducing particles made on the induction of normal λ lysogens: the first is capable of transducing by the insertion of the whole transducing genome into the host chromosome, so its genome must be capable of circularizing; the second transduces less well by insertion—perhaps not at all; if it does not transduce by insertion then its genome need not be linear.

The formation of a transducing genome can be accomplished in three steps: (a) breaking the lysogenic bacterial chromosomes in two places, (b) joining the fragment ends together to form a circular structure, (c) opening the circle (by ter) to form a linear genome. If the resultant structure meets the requirements for λ packaging, it may be formed into a transducing phage, like a bougus λ.

Any meaningful rearrangement of these steps in which step (b) is omitted or delayed leads to the formation of genomes, which are (1) unable to transduce by insertion (because both of its mature ends are unexposed) and (2) are on the average smaller than genomes which are capable of transducing by insertion (so the resultant transducing phage is less dense). Consequence (2) has been confirmed.

We assume that the red function of λ catalyzes the joining of broken DNA molecules to each other. So red is responsible for rehealing the product of (a) back into a lysogenic chromosome and for catalyzing step (b), the healing of fragment ends into a circular structure. The much elevated level of stable transductants on induction of red ⁻ lysogens hereby is explained.