The isolation of a collection of 44 morphologically T1-like phages is described. It is shown that these phages share some similarity with T1 in terms of cross-inactivation with anti-T1 serum, particle proteins and DNA packaging by the headful process. Virion DNA extracted from these phages was treated with T1 packaging extracts and the reaction mixtures were tested for the formation of infectious phage particles. The packaging efficiencies observed varied from about 1 to 100% of that of virion T1 DNA. Phage λ virion DNA was packaged with an efficiency of between 0.01 and 2% (5 × 10 to 3 × 10 p.f.u./µg DNA), the shorter deleted derivative λ L47 being packaged more efficiently than normal length λ CI857 DNA. Virion DNA from phages T3 and T7 was also packaged at an efficiency similar to that for λ. The packaging of T1 DNA requires the presence of the sequence which initiates headful packaging from a concatemeric precursor. The high efficiency of packaging DNA from some of the T1-like phages may indicate the presence of similar packaging sequences. However, in the case of λ L47, which is known not to contain such a sequence, the DNA packaging reaction must occur by a secondary pathway unrelated to the headful mechanism.

Keyword(s): DNA packaging , pac site and phage T1

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