Restriction Modification System

The restriction modification system (RM system) is used by bacteria, and perhaps other prokaryotic organisms to protect themselves from foreign DNA, such as the one borne by bacteriophages. This phenomenon was first noticed in the 1950s. Certain bacteria strains were found to inhibit (restrict) the growth of viruses grown in previous strains. This effect was attributed to sequence-specific restriction enzymes.

Bacteria have restriction enzymes, also called restriction endonucleases, which cleave double stranded DNA at specific points into fragments, which are then degraded further by other endonucleases. This prevents infection by effectively destroying the foreign DNA introduced by an infectious agent (such as a bacteriophage). Approximately one quarter of known bacteria possess RM systems and of those about one half have more than one type of system.

Given that the sequences that the restriction enzymes recognize are very short, the bacterium itself will almost certainly have many of these sequences present in its own DNA. Therefore, in order to prevent destruction of its own DNA by the restriction enzymes, the bacterium marks its own DNA by adding methyl groups to it. This modification must not interfere with the DNA base-pairing, and therefore, usually only a few specific bases are modified on each strand.

Endonucleases cleave internal/non-terminal phosphodiester bonds. Restriction endonucleases cleave internal phosphodiester bonds only after recognising specific sequences in DNA which are usually 4-6 base pairs long, and often palindromic.