Subsystem: Restriction-Modification System

This subsystem's description is:

Restriction–modification (R–M) systems are common in bacteria and are generally believed to provide a barrier against foreign DNA and bacteriophages. In classical R–M systems, foreign DNA is cleaved, or restricted, by endonucleases. Host cell DNA avoids restriction through the methylation, or modification, of certain adenine or cytosine residues in the target sequence. Based on subunit composition, cofactor requirements, and position of the DNA cleavage site, R–M systems have been classified into four distinct groups, namely, type I, type II, type III and type IV.
The type I enzyme is a bifunctional, multisubunit complex containing products of the hsdR, hsdM and hsdS genes (host specificity for DNA). HsdS interacts with the target sequence as a component of the restriction and modification complexes. HsdS is composed of four domains: two variable target recognition domains (TRDs), a central-conserved domain, and a conserved C-terminus. The type I target sequence is asymmetric and composed of two half-sites: a 5' half-site, of 3–4 bp, and a 3' half-site, of 4–5 bp, separated by a non-specific spacer of 6–8 bp. In the HsdS subunit, each TRD recognizes one half-site while the conserved domains are thought to interact with the HsdR and HsdM proteins in the complex.


1.0 - Type I restriction-modification system;

2.0 - Type III restriction-modification system;

3.0 - Type I + Type III restriction-modification system;

x after dot - stands for missing genes in any of variants

For more information, please check out the description and the additional notes tabs, below

Literature ReferencesDiversity within the Campylobacter jejuni type I restriction-modification loci. Miller WG Microbiology (Reading, England) 2005 Feb15699185
DiagramFunctional RolesSubsystem SpreadsheetDescriptionAdditional Notes 

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