Linear DNAs have been prepared from {varphi}X phage and from {varphi}X RF II (double-stranded circular form of {varphi}X DNA, formed during infection and nicked in one or both strands) molecules derived from infection at the restrictive temperature of Escherichia coli ts7, a host mutant with a temperature-sensitive DNA ligase activity. The linear DNA from these phages can be circularized by annealing with fragments of {varphi}X 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 {varphi}X 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
