Mutual exclusion between related phages.

The term mutual exclusion is used to designate an extreme form of interference which often occurs when two phage particles attack the same host cell. The cell liberates numerous particles of one of the parental types and not a single particle of the other parental type (Delbruck and Luria, 1942; Dulbriick, 1945). Mutual exclusion is the rule when the two infecting particles are very dissimilar. However, when they are similar and infect the cell simultaneously, both may multiply. Under these conditions genetic recombination between the two parental types may also take place. Recently French et al. (1951) discovered that infection of an Escherichia coli, strain B, cell with a particle of the related large phages T2, T4, and T6, and of the unrelated phage T6, within two minutes produces a profound modification in the bacterium. This modification is manifested in the fact that T2 particles superinfecting the cell two minutes or later after the first infection are broken down. In the experiments of Lesley et at. (1951) the superinfecting phage was labeled with P32, and its breakdown was tested for by determination of trichloracetic acid soluble label. Up to 50 per cent of the label of the superinfecting phage particles was found in the soluble fraction within a few minutes after its absorption onto bacteria previously infected with another phage of the group mentioned. Although these experiments of Lesley et al. (1951) testify only as to the breakdown of the nucleic acid, they lead one to suspect that the entire phage particle is broken down. This would indeed constitute a very potent mechanism of exclusion, but one of very different characteristics from the case where two very dissimilar phages are involved. To distinguish the two, we will call the first one exclusion by "incompatibility" and this new phenomenon, following Lesley et al., exclusion by "stimulation to breakdown." Incompatibility is a general characteristic of dissimilar phage pairs and is independent of timing relationships. Stimulation to breakdown is produced only by the large phages and requires a time differential between the arrival of the stimulating phage and of the one to be broken down. The discovery of the breakdown of superinfecting phage needs to be supplemented by genetic tests to find out whether the phage whose nucleic acid is broken down is, indeed, unable to contribute genetic markers to the progeny. The present paper presents experiments of this kind, whose results leave no doubt that the breakdown of superinfecting phage is strictly paralleled by exclusion from the progeny of the genetic markers it contains. We are presenting also some experiments in which the stimulating phage alone or both the stimulating and the superinfecting phage have been inactivated by