Restriction Modification Systems: Where They Are and What They Do

Abstract

Definition This review concentrates on restriction-modification (RM) in the context of bacterial genome evolution and how the systems affect bacterial populations. RM systems regulate the entry of foreign DNA into cells. A model of how the systems work is shown in Figure 8-1. Foreign DNA is restricted by a restriction endonuclease that recognizes a specific sequence and cleaves the DNA unless the sequence is protected. Typically, a modification methyltransferase confers protection, by methylating a particular base within the sequence recognized by the restriction enzyme, thereby rendering it resistant to cleavage. Alternatively, however, some restriction enzymes recognize a sequence only when it is methylated. In this case, methylation of a suitable base confers sensitiv-ity to restriction and protection arises from failure to methylate the relevant sequence. Both sorts of restriction can act to limit the transfer of DNA into cells. One key feature of RM is that the systems can be effective only if they are variable and fluid within a bacterial population. Modifications made to foreign DNA escaping restriction are epigenetic, i.e., not heritable. How are they found? Genetic Methods The first RM systems were detected by genetic means, as host factors reducing phage viability in E. coli (Bertani and Weigle, 1953; Luria and Human, 1952). Restriction also acts to reduce the frequency of other forms of genetic exchange in E. coli, including the transfer of chromosomal markers by conjugation or transduction, as well as plasmid transfer (Arber and Linn, 1969; Raleigh, 1992). 78