Nature of φX174 Linear DNA from a DNA Ligase-Defective Host

Abstract

Linear DNAs have been prepared from φX phage and from φX RF II (double-stranded circular form of φX DNA, formed during infection and nicked in one or both strands) molecules derived from infection at the restrictive temperature of Escherichia coli ts 7, a host mutant with a temperature-sensitive DNA ligase activity. The linear DNA from these phages can be circularized by annealing with fragments of φX RF DNA produced by the Haemophilus influenzae restriction nuclease. The circularization experiment indicated that the site of breakage of the linear phage DNAs is not unique nor confined to a particular region of the genome. These linear DNAs were less than 0.1% as infective as circular phage DNA. The linear, positive strand of late RF II DNA, however, is uniquely nicked in the region of the φX genome corresponding to cistron A. Although a low level of infectivity is associated with the linear DNA derived from late RF II, this infectivity appears to be a result of the association of linear positive and linear negative strands during the infectivity assay.