Restriction endonuclease type IV, Mrr (IPR007560)

Short name: Restrct_endonuc_IV_Mrr

Overlapping homologous superfamilies

Domain relationships



There are four classes of restriction endonucleases: types I, II,III and IV. All types of enzymes recognise specific short DNA sequences and carry out the endonucleolytic cleavage of DNA to give specific double-stranded fragments with terminal 5'-phosphates. They differ in their recognition sequence, subunit composition, cleavage position, and cofactor requirements [PMID: 15121719, PMID: 12665693], as summarised below:

  • Type I enzymes (EC: cleave at sites remote from recognition site; require both ATP and S-adenosyl-L-methionine to function; multifunctional protein with both restriction and methylase (EC: activities.
  • Type II enzymes (EC: cleave within or at short specific distances from recognition site; most require magnesium; single function (restriction) enzymes independent of methylase.
  • Type III enzymes (EC: cleave at sites a short distance from recognition site; require ATP (but doesn't hydrolyse it); S-adenosyl-L-methionine stimulates reaction but is not required; exists as part of a complex with a modification methylase methylase (EC:
  • Type IV enzymes target methylated DNA.

This entry represents Mrr, a type IV restriction endonuclease involved in the acceptance of modified foreign DNA, restricting both adenine- and cytosine-methylated DNA. Plasmids carrying HincII, HpaI, and TaqI R and M genes are severely restricted in Escherichia coli strains that are Mrr+ [PMID: 1650347]. Mrr appears to be the final effector of the bacterial SOS response, which is not only a vital reply to DNA damage but also constitutes an essential mechanism for the generation of genetic variability that in turn fuels adaptation and resistance development in bacterial populations [PMID: 16313623]. Mrr possesses a cleavage domain that is similar to that found in type II restriction enzymes, however it has an unusual glutamine residue at the central position of the (D/E)-(D/E)XK hallmark of the active site [PMID: 11313145].

GO terms

Biological Process

GO:0009307 DNA restriction-modification system

Molecular Function

GO:0003677 DNA binding
GO:0004519 endonuclease activity

Cellular Component

No terms assigned in this category.

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.