CHARACTERIZATION OF PLASMIDS ENCODING TYPE-II RESTRICTION - MODIFICATION SYSTEMS, ISOSCHIZOMERS OF SSOII

Five strains of Enterobacteriaceae family have been revealed that have restriction-modification systems of type II, isoschizomers of SsoII. The genes coding for the restriction endonucleases and DNA methyltrans ferases are located on plasmids of two types. These plasmids differ in size, capacity for mobilization by conjugative plasmids, and structur e of locus cer. Recombinant two-plasmid strains were obtained containi ng plasmids with genes for Ecl18kI and EcoRII restriction endonuclease s in the presence of DNA methyltransferase Ecl18kI. The stability of t he two-plasmid system with the ecoRIIR and ecl18kIM genes implies simi lar mechanisms of concerted regulation of gene expression in the restr iction-modification systems Ecl18kI and EcoRII. Genes for Ecl18kI and SsoII restriction-modification systems have essentially the same seque nce.