Chapter 4 Host-controlled Variation

Abstract

About twenty years ago a number of similar observations were made in work with various bacteriophage species. They showed that the host range of a given phage preparation depended on the bacterial strain in which the phage had last propagated. This effect was called host-controlled variation to distinguish its host-dependent and thus genetically unstable nature from persistent hereditary changes such as are found in host-range mutants. Many examples now are known to reflect the phenomena described in this chapter: strain-specific restriction and modification of DNA.

Host-controlled variation was one of the first properties of bacteriophage λ to be studied. The natural host of λ is Escherichia coli K12 (Lederberg and Lederberg, 1953). Phage grown on it is called λ·K. E. coli C is also a host of phage λ and it produces by definition λ·C. Bertani and Weigle (1953) observed that λ·C grows on E. coli K12 with the low efficiency of 2 × 10⁻⁴, whereas it grows well on strain C. Phage λ·K, on the other hand, grows well on both strains (Fig. 1). Bertani and Weigle showed that the only measurable difference between λ·K and λ·C was the inability of λ·C to grow on K12; the two phages were alike in all other tested properties. These authors also showed that λ·K was not a genetic mutant of λ·C and they concluded that the host strains were responsible for the difference between the two phage variants.

These conclusions have since been confirmed: restriction and modification are host functions and...