Editing Restriction enzyme (section)

=== Type l ===
Type I restriction enzymes were the first to be identified and were first identified in two different strains (K-12 and B) of ''[[Escherichia coli|E. coli]]''.<ref name="pmid10839821">{{cite journal | vauthors = Murray NE | title = Type I restriction systems: sophisticated molecular machines (a legacy of Bertani and Weigle) | journal = Microbiology and Molecular Biology Reviews | volume = 64 | issue = 2 | pages = 412–34 | date = June 2000 | pmid = 10839821 | pmc = 98998 | doi = 10.1128/MMBR.64.2.412-434.2000 }}</ref> These enzymes cut at a site that differs, and is a random distance (at least 1000 bp) away, from their recognition site. Cleavage at these random sites follows a process of DNA translocation, which shows that these enzymes are also molecular motors. The recognition site is asymmetrical and is composed of two specific portions—one containing 3–4 nucleotides, and another containing 4–5 nucleotides—separated by a non-specific spacer of about 6–8 nucleotides. These enzymes are multifunctional and are capable of both restriction digestion and modification activities, depending upon the methylation status of the target DNA. The cofactors [[S-Adenosyl methionine]] (AdoMet), hydrolyzed adenosine triphosphate ([[Adenosine triphosphate|ATP]]), and [[magnesium]] (Mg<sup>2+</sup>) [[ion]]s, are required for their full activity. Type I restriction enzymes possess three subunits called HsdR, HsdM, and HsdS; HsdR is required for restriction digestion; HsdM is necessary for adding [[methyl]] groups to host DNA (methyltransferase activity), and HsdS is important for specificity of the recognition (DNA-binding) site in addition to both restriction digestion (DNA cleavage) and modification (DNA methyltransferase) activity.<ref name="pmid8336674"/><ref name="pmid10839821"/>