Transcription regulation in bacterial restrictionmodification (RM) systems is an important process, which provides coordinated expression levels of tandem enzymes, DNA methyltransferase (MTase) and restriction endonuclease (RE) protecting cells against penetration of alien DNA. The present study focuses on (cytosine-5)-DNA methyltransferase Ecl18kI (M.Ecl18kI), which is almost identical to DNA methyltransferase SsoII (M.SsoII) in terms of its structure and properties. Each of these enzymes inhibits expression of the intrinsic gene and activates expression of the corresponding RE gene via binding to the regulatory site in the promoter region of these genes. In the present work, complex formation of M.Ecl18kI and RNA polymerase from Escherichia сoli with the promoter regions of the MTase and RE genes is studied. The mechanism of regulation of gene expression in the Ecl18kI RM system is thoroughly investigated. M.Ecl18kI and RNA polymerase are shown to compete for binding to the promoter region. However, no direct contacts between M.Ecl18kI and RNA polymerase are detected. The properties of M.Ecl18kI and M.SsoII mutants are studied. Amino acid substitutions in the N-terminal region of M.Ecl18kI, which performs the regulatory function, are shown to influence not only M.Ecl18kI capability to interact with the regulatory site and to act as a transcription factor, but also its ability to bind and methylate the substrate DNA. The loss of methylation activity does not prevent MTase from performing its regulatory function and even increases its affinity to the regulatory site. However, the presence of the domain responsible for methylation in the M.Ecl18kI molecule is necessary for M.Ecl18kI to perform its regulatory function.