FACTORS AFFECTING PHAGE-RESTRICTING ABILITY IN RHIZOBIUM LEGUMINOSARUM STRAIN L4

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Abstract
Rhizobiophage oL5 is modified, by growth in Rhizobium legumlnosarum strain L25, to the oL5.L25 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 oL5 as helper phage increased EOP on L4 up to 200-fold, depending on time and sequence of infection. Multiple (cooperative) infection with oL5.L25 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.