Factors Affecting Phage-Restricting Ability in Rhizobium Leguminosarum Strain L4

Rhizobiophage [Formula: see text] is modified, by growth in Rhizobium leguminosarum strain L25, to the [Formula: see text] form, which has a low plating efficiency on host strain L4. Alteration of plating efficiency (EOP) was used as a means of studying the nature of phage-restricting ability in L4 cells. Mixed infection involving another phage or the unrestricted form of [Formula: see text] as helper phage increased EOP on L4 up to 200-fold, depending on time and sequence of infection. Multiple (cooperative) infection with [Formula: see text] or sublethal heat treatment of cells increased EOP by about 1000-fold. Growth of cells after they were heated resulted in fairly rapid recovery of restricting ability. Ultraviolet light irradiation of cells induced a lower order of EOP enhancement (7- to 10-fold), and loss and recovery of restricting ability appeared to occur more slowly. Some evidence of an interaction between de-restricting treatments was noted for irradiation and multiple infection. Rapid inactivation of the restricted phage genome after injection was indicated in experiments in which de-restricting treatments were applied belatedly after infection. The results and their implications concerning the nature of restricting ability are compared with those of other phage–host systems.