strain adsorbs phage λ very efficiently but the phage does not form plaques on this strain. In a very small fraction (10) of the infected cells the phage grows and produces small bursts of progeny phage also unable to form plaques on strain strain is lysogenic for a temperate phage, φ, related to phage 2. Non-restricting hosts for phage λ became restricting hosts when made lysogenic for φ. When P-labelled λ adsorbed to restricting φ lysogenic hosts, > 20% of the P become acid-soluble shortly after infection. No φ specific modification was carried by the small number of λ phages which escaped this restriction process. It is concluded that φ controls a host-restriction mechanism but not a host-modification process, and in parallel with other examples of host-controlled restriction and modification can be represented as or °. λ mutants have been isolated which escape this restriction and which form plaques on strain and φ lysogenic strains with an efficiency of 1.0. With these mutants a -specific host modification controlled by the genome of strain was demonstrated. Mixed infection experiments with restricted λ and unrestricted λ showed that that restricted phage did not block the growth of the unrestricted mutant nor did the mutant permit the restricted phage to grow. In addition it was shown that λ obtained from bacteria mixedly infected with λ and λ was still unable to grow in restricting hosts and λ similarly obtained from mixedly infected bacteria still retained its ability to grow on restricting hosts. It is concluded that there is a nucleotide sequence in the DNA of phage λ which, when λ infects a restricting host, is specifically recognized by the restriction mechanism controlled by the φ. The mutation to λ involves an alteration to this sequence such that it is no longer recognized by the restriction mechanism of the φ.

Mutants of φ were isolated not restrictive for phage λ.


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