Type I Restriction and Modification Systems

DNA Methylation

All the known Type I restriction endonucleases are N-6 adenosyl methyltransferases. While the enzymes are all capable of methylating unmethylated DNA, however, the preferred substrate is hemimethylated DNA. Hemimethylated DNA, for instance produced following DNA replication of fully modified DNA, induces the restriction endonuclease to methylate the unmethylated strand. Unmethylated DNA induces the restriction activity of the enzyme and DNA cleavage follows. The enzyme dissociates from fully modified DNA without any further activity. DNA cleavage is not a simple reaction, and the other enzymatic properties of the endonuclease become apparent during this process.

Most DNA methyltransferases are monomeric enzymes that methylate single bases one at a time. However, the study of Type I Restriction-Modification (R-M) enzymes can add significantly to our understanding of these processes since they are unusual enzymes that must communicate the methylation status of two bases. These enzymes are multifunctional (acting as endonucleases, DNA methyltransferases, ATPases and DNA translocators), multisubunit enzymes and must control the opposing functions of DNA cleavage (restriction) and DNA methylation (modification). The requirement of Type I R-M enzymes to determine the methylation status of two specific bases in the recognition sequence reflects the multifunctional nature of the enzyme. Detection of unmethylated DNA (2-bases unmethylated) triggers a conformational change in the R-M enzyme, which leads to translocation of DNA and subsequent cleavage. Therefore, the enzyme must be able to read the methylation status of a single-base (perhaps using base-flipping as for other DNA methyltransferases) and then "remember" this methylation status while it reads the methylation status of the second base in the recognition sequence. This information must then be communicated to the R-M enzyme allowing a switch to restriction activity if both sites are unmethylated.