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Cells of S. pneumoniae contain either DpnI, a restriction endonuclease that
cleaves only the methylated DNA sequence 5'-GmeATC-3', or DpnII, which cleaves
the same sequence when not methylated. A chromosomal DNA segment containing
DpnII 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 DpnI cells, recombination
during chromosomal facilitation of its establishment substituted genes encoding
the DpnI endonuclease and another protein in place of the DpnII genes. DNA
hybridization and sequencing showed that the DpnI and DpnII 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
nonhomologous and mutually exclusive cassettes that can be inserted into a
particular point in the chromosome of S. pneumoniae on the basis of neighboring
homology.
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