The regions of bacteriophage Mu involved in host cell killing were determined by infection of a λ-immune host with 12 λpMu-transducing phages carrying different amounts of Mu DNA beginning at the left end. Infecting λpMu phages containing 5.0 (±0.2) kb or less of the left end of Mu DNA did not kill the λ-immune host, whereas λpMu containing 5.1 kb did kill, thus locating the right end of the kil gene between approximately 5.0 and 5.1 kb. For the Kil+phages the extent of killing increased as the multiplicity of infection (m.o.i.) increased. In addition, killing was also affected by the presence of at least two other regions of Mu DNA: one, located between 5.1 and 5.8 kb, decreased the extent of killing; the other, located between 6.3 and 7.9 kb, greatly increased host cell killing. Killing was also assayed after λpMu infection of a λ-immune host carrying a mini-Mu deleted for most of the B gene and the middle region of Mu DNA. Complementation of mini-Mu replication by infecting B+λpMu phages resulted in killing of the λ-immune, mini-Mu-containing host, regardless of the presence or absence of the Mu kil gene. The extent of host cell killing increased as the m.o.i. of the infecting λpMu increased, and was further enhanced by both the presence of the kil gene and the region located between 6.3 and 7.9 kb. These distinct processes of kil-mediated killing in the absence of replication and non-kil-mediated killing in the presence of replication were also observed after induction of replication-deficient and kil mutant prophages, respectively. © 1984.
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