Virus adaptation to host cells: the non-classical modification of phage T3

Bacterial virus T3 undergoes host-controlled modification which is not based on "classical" processes of DNA modification and restriction. The adsorption and thus the growth of T3 on Escherichia coli W cells (E. coli K12 derivative) decisively depends on the host strain on which the virus was previously propagated. Depending on the modification conferred to the virus by its last host, its efficiency of plating (e.o.p.) on E. coli W varies by six orders of magnitude between 10(-7) and 10(-1). This does not reflect the appearance of T3 host-range mutants, but a fully reversible modification of genotypically unchanged T3 wild-type phage. The behaviour of T3 in the described host system constitutes a second case of so-called non-classical modification and restriction (KRUGER et al. 1977, Molec. gen. Genet. 153, 107-110) of bacteriophages. Non-classical modification (protein modification) is additive to and independent of DNA modification and restriction as demonstrated with the ocr- phage T3/R7. - Furthermore, our results suggest that the adsorption specificity of T3 is determined by at least two independent genetic factors; in both of these factors T3 differs from T7.