Zygotic induction of the rac locus can cause cell death in E. coli

Conjugational transfer of the rac locus of E. coli K-12 into a Rac- recipient strain (i.e. rac+ X rac-) results in the killing of a majority of the recipient cells. The efficiency of killing depends somewhat on the plating medium, and can be as high as 98%. The killing is not observed in the rac+ X rac+, rac- X rac- or rac- X rac+ configurations. The rac locus, which has the properties of a cryptic prophage, may carry a function analogous to the kil function of bacteriophage lambda, or may instead cause killing by some replication related process.