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Type I restriction modification DNA specificity domain This domain is also known as the target recognition domain (TRD). Restriction-modification (R-M) systems protect a bacterial cell against invasion of foreign DNA by endonucleolytic cleavage of DNA that lacks a site specific modification. The host genome is protected from cleavage by methylation of specific nucleotides in the target sites. In type I systems, both restriction and modification activities are present in one heteromeric enzyme complex composed of one DNA specificity subunit (this family), two modification (M) subunits and two restriction (R) subunits.
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