Normal expression of the viral gene N interferes with growth of bacteriophage λ in Escherichia coli 15T^-

Summary

Growth of λ and of some lambdoid phages is considerably inhibited on strain 3057 derived from E. coli 15T^-. Mutants of λ which overcome this inhibition map in gene N. Some of these λhty mutants are temperature sensitive for growth on E. coli K12. Thus plating of λ on strain 3057 allows one to isolate temperature sensitive N mutants. The λhty mutants produce less than normal N activity as judged by their low efficiency of plating on a nus ^- host and by the extended latent period of some of them on normal hosts. The inability of strain 3057 to propagate λ can be at least partially reversed by addition of thymidine to the medium and the growth difference between λhty and λ in 3057 increases with decreasing thymidine concentration. The amount of DNA produced by λ in 3057 at low thymidine concentration is lower than that produced by λhty under the same conditions. Only a small percentage of the DNA produced by λ in 3057 is packaged into viable phage particles. This suggests that λ not only produces less DNA in 3057 than λhty but that an important part of the λ DNA in 3057 is in a form which can not be packaged or which is noninfective for other reasons. A hypothesis is discussed that hty mutations enable λ to grow on E. coli 15T^- at low thymidine concentration because they lead to reduction in the number of single strand nicks in the λ DNA by reducing the intracellular endonuclease activity. Under permissive conditions conditional lethal N mutants are favored for growth on 3057 over λN ⁺ which confirms the idea that N activity or the activity of a gene under N control interferes with λ growth in 3057 at low thymidine concentration.