DNA modification and restriction.

Publisher Summary This chapter discusses the interaction of restriction endonucleases with double-stranded DNA molecules at specific sites leading to cleavage of the DNA into a number of fragments. The specificity of this interaction is thought to depend on the recognition by the enzyme of a particular sequence of base-pairs on the substrate DNA. Restriction endonucleases are found in many bacterial strains as products of genes carried either on the bacterial chromosome or on plasmid DNA. Among enzymes obtained from independent sources, each usually shows its own specificity of interaction. The chapter describes that a bacterial strain can protect its own DNA from cleavage by its restriction endonucleases. This protection is brought about by site-specific methylation of the DNA, for which another activity, the DNA modification methylase, is responsible. Both endonuclease and methylase are thought to recognize the same base sequences on their substrate DNA. Each independent system of restriction and modification activities would then recognize its own particular target on the DNA. Therefore, the modification given by a particular methylase protects the DNA only against restriction by the correlated endonuclease. The chapter also discusses that mostly for reasons of a historical and practical nature, laboratory strains such as Escherichia coli K12 and B or Haemophilus influenzae are widely used in the experimental investigation of DNA restriction and modification, although many other bacteria are either known or are likely to have restriction and modification systems.