Formation of defective bacteriophage particles by fr amber mutants

Abstract

The object of the present work was to study the events which take place after infection of the non-permissive host with the fr amber mutant su m . In this host, defective particles are formed which have a lower buoyant density than fr bacteriophage and which contain only 70% of the normal amount of RNA. This RNA consists of several pieces. The protein coat of these “light, defective particles”, as examined by electron microscopy, appears to be the same as that of fr phages. The end group amino-acid analysis further suggests that the amber mutation does not affect the coat protein. The RNA in the light defective particles is not infectious; nevertheless the mutant su m initiates the synthesis of infectious RNA in normal amounts. If a bacterial strain which is deficient in cell-wall ribonuclease is used as a non-permissive host, so-called “heavy, defective particles” are obtained. Their buoyant density is the same as that of fr phages, but their sedimentation coefficient is 69 s instead of 79 s. On treatment with ribonuclease, the buoyant density of these particles is decreased to that of light, defective particles. From this the conclusion is drawn that in the non-permissive host the assembly of normal RNA and normal coat protein into nucleoprotein particles is a defective process which does not produce viable fr phages. A phage-specific protein factor is assumed to be essential for establishing or stabilizing the conformation of the RNA.