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Concept
Restriction modification system
Summary
The restriction modification system (RM system) is found in bacteria and other prokaryotic organisms, and provides a defense against foreign DNA, such as that borne by bacteriophages.
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.
As the sequences recognized by the restriction enzymes are very short, the bacterium itself will almost certainly contain some within its genome. In order to prevent destruction of its own DNA by the restriction enzymes, methyl groups are added. These modifications 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. They do so only after recognising specific sequences in DNA which are usually 4-6 base pairs long, and often palindromic.
The RM system was first discovered by Salvatore Luria and Mary Human in 1952 and 1953. They found that bacteriophage growing within an infected bacterium could be modified, so that upon their release and re-infection of a related bacterium the bacteriophage’s growth is restricted (inhibited) (also described by Luria in his autobiography on pages 45 and 99 in 1984). In 1953, Jean Weigle and Giuseppe Bertani reported similar examples of host-controlled modification using different bacteriophage system. Later work by Daisy Roulland-Dussoix and Werner Arber in 1962 and many other subsequent workers led to the understanding that restriction was due to attack and breakdown of the modified bacteriophage’s DNA by specific enzymes of the recipient bacteria. Further work by Hamilton O.
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Related concepts (8)
Restriction modification system
The restriction modification system (RM system) is found in bacteria and other prokaryotic organisms, and provides a defense against foreign DNA, such as that borne by bacteriophages. 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).
Bacteria
Bacteria (bækˈtɪəriə; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere.
DNA methylation
DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription. In mammals, DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis.