FIG. 8.

Phage immunity test. λ phages of wild-type (λ) and ultravirulent (λvir) phenotypes (2) were assayed on lawns of bacteria by spot tests at increasing phage titers (from left to right, 10², 10³, and 10⁴ PFU per spot). Bacteria were transformed with plasmids containing the λ repressor fused to the actual or putative dimerization elements indicated below. “none,” CSH50 E. coli cells transformed with pcI-NH₂ (expressing the NH₂-terminal domain of λ repressor without any putative dimerization element); NirA, the same cells transformed with pcI-NDD, containing the putative NirA dimerization domain; Rop, a positive control transformed with pC132, expressing a cI-Rop fusion (the Rop protein efficiently dimerizes to form a bundle of four antiparallel helices); Rop*, transformed with pC135 expressing a cI-Rop* fusion with a mutated Rop protein unable to dimerize (7).