Practice question

Host specificity determinant genes hsdR, hsdM, and hsdS encode components of Type I restriction-modification systems first characterized in Escherichia coli strains K and B. hsdS confers sequence specificity via two target recognition domains, hsdM provides N6-adenine methylation and N4-cytosine methylation activity, and hsdR provides ATP-dependent restriction endonuclease and translocase functions. The R2M2S1 pentamer can switch between restriction and modification modes regulated by S-adenosylmethionine levels. This genetic organization distinguishes Type I from Type II enzymes encoded by independent genes and Type III possessing res and mod genes.