Genetic basis of the complementary Dpnl and Dpnll restriction systems of S. pneumoniae: An intercellular cassette mechanism

Cells of S. pneumoniae contain either Dpnl, a restriction endonuclease that cleaves only the methylated DNA sequence 5′-GmeATC-3′, or Dpnll, which cleaves the same sequence when not methylated. A chromosomal DNA segment containing Dpnll genes was cloned in S. pneumoniae. Nucleotide sequencing of this segment revealed genes encoding the methylase and endonuclease and a third protein of unknown function. When the plasmid was introduced into Dpnl cells, recombination during chromosomal facilitation of its establishment substituted genes encoding the Dpnl endonuclease and another protein in place of the Dpnll genes. DNA hybridization and sequencing showed that the Dpnl and Dpnll segments share homology on either side but not between themselves or with other regions of the chromosome. Thus, the complementary restriction systems are found on non-homologous and mutually exclusive cassettes that can be inserted into a particular point in the chromosome of S. pneumoniae on the basis of neighboring homology.