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.
Os encimas de restrición de tipo I posúen tres subunidades chamadas HsdR, HsdM, e HsdS; a subunidade HsdR requírese para a restrición; a HsdM é necesaria para engadir grupos metilo ao ADN do hóspede (actividade metiltransferase) e a HsdS é importante para a especificidade do sitio de recoñecemento (de unión ao ADN) ademais de para as actividades de restrición (clivaxe do ADN) e modificación (ADN metiltransferase).