Practice question

Type I restriction enzymes are hetero-oligomeric complexes with stoichiometry R2M2S1 incorporating hsdR restriction, hsdM modification, and hsdS specificity subunits. They recognize bipartite asymmetric sequences containing non-specific spacers and translocate DNA using ATP hydrolysis before cleaving at variable distances up to a thousand base pairs from recognition site. Catalysis requires magnesium ions for phosphodiester hydrolysis and S-adenosylmethionine as allosteric activator promoting conformational changes and serving as methyl donor. Due to random cutting position, Type I enzymes are unsuitable for precise cloning but function in bacterial defense.