Characterization of bacteriophages infecting Streptomyces erythreus and properties of phage-resistant mutants

Three bacteriophages infecting Streptomyces erythreus, called G3, G4 and G5, were isolated and characterized. They contain double-stranded linear DNA molecules with cohesive ends. The restriction map of G3 DNA (48 kilobases long) for four restriction endonucleases and that of G4 DNA (43 kilobases long) for seven restriction endonucleases are reported. Restriction analysis and hybridization experiments showed that G3 and G4 share little DNA homology, while G4 and G5 are apparently identical except for an additional EcoRI site present in G5. The region containing this EcoRI site has been mapped on G4 DNA. Microbiological and serological data showed that G5 is very similar to G4. G3- and G4-resistant mutants of S. erythreus PS1 were isolated. The screening of phage-resistant mutants showed a high frequency of strains with increased erythromycin production. The mechanism of phage resistance of strain PS3 (G3 resistant) and of strain PS16 (G4 resistant) was examined. The DNA of the resistant strains contains no phage DNA, ruling out lysogeny as a cause of phage resistance. Transfection of strains PS1, PS3, and PS16 with DNA of the three phages showed the same efficiency, indicating that resistance is at the level of the bacterial wall.