SUMMARY: We analysed the physical structure of the DNA of øFSW, which is a temperate phage of S-1. A circular restriction map of the virion DNA has been constructed with three restriction endonucleases, HI, I and I. Other data indicated that the phage genome was circularly permuted. In lysogens, the DNA of the prophage was found to be linearized at a specific site and integrated into a specific locus of the host genome, with the same orientation in each case, as evidenced by Southern filter hybridization. We compared the physical structure of øFSW with its three virulent mutants. One of them had a restriction map indistinguishable from that of øFSW and two of them contained host-derived DNA sequence(s) in a specific region of the øFSW genome (V-region). The prophage integration site was mapped on a different segment of the phage genome to the V-region. Derivation of virulent mutants from øFSW is discussed in relation to the physical structure of the phage genome.


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