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Evidence of Horizontal Transfer of theEcoO109I Restriction-Modification Gene to Escherichia coli Chromosomal DNA

ABSTRACT A DNA fragment carrying the genes coding for EcoO109I endonuclease and EcoO109I methylase, which recognize the nucleotide sequence 5′-(A/G)GGNCC(C/T)-3′, was cloned from the chromosomal DNA of Escherichia coli H709c. TheEcoO109I restriction-modification (R-M) system was found to be inserted between the int and psu genes from satellite bacteriophage P4, which were lysogenized in the chromosome at the P4 phage attachment site of the corresponding leuX gene observed in E. coli K-12 chromosomal DNA. Thesid gene of the prophage was inactivated by insertion of one copy of IS21. These findings may shed light on the horizontal transfer and stable maintenance of the R-M system.