Transfer of the genes for the StyLTI restriction-modification system of Salmonella typhimurium to strains lacking modification ability results in death of the recipient cells and degradation of their DNA.

The genes encoding the restriction-modification system StyLTI of Salmonella typhimurium were inserted in vivo into the conjugative plasmid pULB21. This allowed us to transfer the StyLTI genes at a very high frequency and to monitor the fate of recipient cells after mating. Transfer of the StyLTI restriction and modification genes into a modificationless recipient was lethal and resulted in degradation of the cell's DNA. This indicates that, in contrast to any other known restriction-modification systems, StyLTI cannot be established after horizontal transfer into a naive host.