strain LR25 carries three plasmids with molecular weights of 11.2, 18.3 and 107 x 10 as determined by electron microscopy. A number of phages propagated on ATCC 607 were tested for their ability to infect strain LR25. Phage JF2 gave an efficiency of plating of 10 on strain LR25, but phage JF2 propagated on strain LR25 infected strain LR25 and with equal high efficiency. This indicated the presence of a restriction-modification (R-M) system in strain LR25 that was not present in Strain LR25 was grown in the presence of acriflavine to eliminate the plasmids and tested for sensitivity to phage JF2. One of forty colonies was found to be R-M-deficient. This strain, designated strain LR163, lacks the three plasmids present in strain LR25. The results indicate that the R-M system is plasmid-coded. Strain LR163 was sensitive to several phages to which strain LR25 was resistant and for which we were unable to isolate modified phage. This suggests that some plasmid-coded function in addition to restriction is involved. An R-M system was also demonstrated in strain LR131 using phage JF1. This strain does not carry plasmids.


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