<P> 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 . It 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 systems . 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 . As reviewed by Daniel Nathans and Hamilton O. Smith in 1975, this work resulted in the discovery of the class of enzymes now known as restriction enzymes . When these enzymes were isolated in the laboratory they could be used for controlled manipulation of DNA, thus providing the foundation for the development of genetic engineering . Werner Arber, Daniel Nathans, and Hamilton Smith were awarded the Nobel Prize in Physiology or Medicine in 1978 for their work on restriction - modification . </P> <P> 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 . </P>

What are produced by bacteria and cut dna at specific points