Cell toxicity caused by products of the pL operon of bacteriophage lambda

Abstract

Induction of a λ prophage causes the death of the host cell even in the absence of phage replication and lytic functions due to expression of functions from the λpL operon. We genetically modified the λ prophage to determine which λpL operon functions were involved in cell killing. Viability assays and flow cytometry were used to monitor cell death and filamentation. The kil gene was shown to cause cell death and filamentation as described previously. Another killing activity was mapped within the pL operon to the gam gene. Inspection of the DNA sequence showed that there are two possible translation start points for both kil and gam. In both cases, the shorter of the two possible products could cause cell killing. The shorter products were also sufficient for the known filamentation and recombination activities of the respective Kil and Gam functions. The expression level of the pL operon is down-regulated by Cro repressor. In the absence of Cro, higher pL expression levels allow either Kil or Gam to be lethal or growth inhibitory, whereas at lowered expression in Cro-repressed conditions, only Kil is lethal. The filamentation function of Kil and recombination activity of Gam are unaffected at Cro-repressed levels of expression.