Fig 1. Examples of defense systems discussed in the text.

Homologous recombination via the RecBCD pathway can defend from MGEs because RecBCD is a powerful exonuclease that degrades linear double-stranded DNA (but is inactivated by some phage encoded proteins, like Gam). Epigenetic modifications are the basis of many defense systems that can be counteracted by phage proteins, like Ocr in phage T7, or by epigenetic modification of phage DNA. Abortive infection is a costly defense strategy that induces cell growth arrest or death when a threat to the cell is detected. Adaptive immunity by CRISPR/Cas systems targets MGEs with specific DNA sequences, while allowing other exogenous DNA to remain in the cell and eventually recombine with the chromosome. CRISPR/Cas, clustered regularly interspaced short palindromic repeats/CRISPR associated protein; MGE, mobile genetic element.