Ecl18kI is a type II restriction-modification system isolated from Enterobacter cloaceae 18kI strain. Genes encoding Ecl18kI methyltransferase (M.Ecl18kI) and Ecl18kI restriction endonuclease (R.Ecl18kI) have been cloned and expressed in Escherichia coli. These enzymes recognize the 5'.../CCNGG...3' sequence in DNA; M.Ecl18kI methylates the C5 carbon atom of the inner dC residue and R.Ecl18kI cuts DNA as shown by the arrow. The restriction endonuclease and the methyltransferase were purified from E. coli B834 [p18Ap1] cells to near homogeneity. The restriction endonuclease is present in the solution as a tetramer, while the methyltransferase is a monomer. The interactions of M.Ecl18kI and R.Ecl18kI with 1,2-dideoxy-D-ribofuranose containing DNA duplexes were investigated. The target base flipping-out mechanism is applicable in the case of M.Ecl18kI. Correct cleavage of the abasic substrates by R.Ecl18kI is accompanied by non-canonical hydrolysis of the modified strand.