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Restriction-Modification Systems of Bacteria The most widely recognizable enzymes that are used in molecular genetics are restriction enzymes. These enzymes are part of the restriction-modification system that bacterial species use to prevent foreign organisms from overtaking their cells. Presumably, each species has one or more of these systems. These syste ms consist of a restriction enzyme that cleaves DNA at a specific sequence and a methylase that protects the host DNA from being cleaved. Let's look at one of these systems in the E. coli. The restriction enzyme site is: *m 5' - G A A T T C - 3' (*Remember the enzyme will not 3' - C T T A A G - 5' cut if the 3' A is methylated.) m* The restriction enzyme that cuts this site is called EcoRI, and it cuts between the G and A. This site is protected in the bacteria by the action of the enzyme EcoRI methylase which adds a methyl group to the 3'-adenine. The DNA that is cut at the EcoRI site will have the following "sticky" ends. 5' - G - 3' 5' - A A T T C - 3' 3' - C T T A A - 5' 3' - G - 5' Invading viral DNA will not be methylated and can be cut by the restriction enzyme. Thus it is said that foreign DNA proliferation is restricted in the cell by the restriction enzyme, and bacterial DNA is modified by the methylase to prevent cleavage by the restriction enzyme. Copyright © 1997. Phillip McClean