FIG. 1.

The primary determinant of Mx8 superinfection immunity, imm, lies at the right end of EcoRI-A. The physical map of phage Mx8 DNA shows restriction sites for EcoRI (R), NheI (N), and Sau3AI (S); not all XhoI (X) sites are shown. EcoRI fragments are labeled A through H, in order of decreasing size. The regions of Mx8 DNA subcloned into plasmids are shown as rectangles below the physical map. Vector pPLH343 carries the 5.8-kb XhoI subfragment of Mx8 DNA from EcoRI fragment B with the int-attP region. Mx8 forms clear plaques on DZ1(pAY45); on all other hosts, Mx8 forms turbid plaques. On hosts DZ1(pAY30), DZ1(pAY31), DZ1(pAY55), DZ1(pAY50), and DZ1(pAY54), virulent mutants in the wild-type stock of Mx8 form clear plaques. Derivatives of the permissive M. xanthus strain DZ1 were grown to exponenial density, and the EOP of Mx8 was measured on each derivative relative to that on nonlysogenic DZ1. Combinations of inserts in plasmids that confer superinfection immunity are shown as open rectangles (EOP < 10⁻⁴); inserts that do not are shown as filled rectangles (EOP = 0.3 ± 0.1). The sequenced 9.5-kb immunity region is shown as a shaded bar.