The DNA-containing phages neutralizing with anti-MS2 serum have been detected in MS2-inducco bacterial, cultures, in the cells containing the recombinant plasmid as well as in preparations of transducing lambda and PI phages. The identity of all these new developed bacteriophages permits to suppose the common mechanism of their origin.

Sensitive cells of Escherichia coli AB 259 Hfr 3000 infected with RNA-containing phage MS2 produce phage particles and simultaneously continue to divide, thereby segregating sensitive cells capable of sustaining new cycles of infection. Multiplication of phage in sensitive cells gives rise to phage-resistant forms in the progeny of these cells. It is shown that this phenomenon is due not to selection of pre-existing phage-resistant mutants, but is instead the result of interaction between the phage and the cell. Unlike ordinary spontaneous or chemically induced E. coli mutants, MS2-induced phage-resistant cells are genetically unstable forms. In the course of reproduction they segregate new MS2-resistant forms with more highly expressed variations in the region encoded by the sex factors. Cells of the two final forms of MS2-induced mutants also produce a new type of phage. This new type constitutes DNA-containing forms which, however, are neutralized by anti-MS2serum. The segregation of these forms serves to confirm that the genetic substance of the RNA-containing bacteriophage is capable of being expressed as a component of the DNA-containing structure.

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pL34 recombinant plasmid coding MS2-like RNA synthesis in plasmid-containing cells has been studied using physical mapping and blot-hybridisation with P 32-labelled pMS27 plasmid Iragmenis including a DNA copy of MS2 phage RNA. The authors have deter mined the size of a fragment containing MS2-like sequence and its localisation inside the hybrid plasmid.