Intact ribonucleic acid from defective particles of bacteriophage R17

The growth of one group of amber mutants of R17 (class A) in non-permissive hosts results in a burst of bacteriophage-like particles, which are defective in that they are non-infectious. Here studies of particles produced by the growth of wild-type R17 and four class A mutants on AB301, a non-permissive Hfr Escherichia coli strain, and on RNase(-)10, an RNase I-deficient derivative of AB301, are described. Defective particles grown on AB301 contain degraded, non-infectious RNA and are on the average less dense than wild-type phage; particles grown on RNase(-)10 contain intact, infectious RNA and exhibit normal density in cesium chloride gradients, but are nonetheless non-infectious as phage. In addition, the RNA in purified defective particles grown on RNase(-)10 is partially susceptible to nuclease attack, whereas RNA in wild-type phage is not. The studies demonstrate that the presence of degraded RNA in defective particles of R17 results from the action of RNase I during the purification of the particles. The possible function of the A protein of R17 is considered in the light of the results obtained.