2001 Fred Griffith review lecture. Immigration control of DNA in bacteria: self versus non-self.

Bacteria commonly endow their DNA with an identity mark. When DNA is transferred from one bacterium to another strain of the same species, DNA that lacks the identification mark of the recipient strain is recognized as ‘ foreign’ rather than ‘self ’. Foreign DNA is commonly degraded. The first evidence for this discriminatory process was the demonstration of a barrier, albeit incomplete, to the productive infection of Escherichia coli strain K-12 by bacteriophage λ previously propagated in either E. coli strain C or E. coli strain B (Bertani & Weigle, 1953). Much later it was proven that the growth of phages in E. coli K-12 can be ‘restricted’ by an endonuclease, a restriction enzyme (EcoKI), which attacks foreign DNA (Meselson & Yuan, 1968; Linn & Arber et al., 1969). Occasionally phages escape restriction and they, like the resident bacterial chromosome, acquire a protective identification mark from a strain-specific modification enzyme that methylates defined bases within a specific target sequence (Arber & Dussoix, 1962; Smith et al., 1972). This sequencespecific modification identifies the immediate provenance of bacterial, or phage, DNA (Fig. 1).