*  HsdS    -    DNA specificity :    two variable and two conserved regions

 

*  HsdM    -    modification :         AdoMet binding site, catalytic site for DNA methylation

 

*  HsdR    -    restriction :             ATP binding site, DEAD box protein

HsdM and HsdS are able to produce an independent
methylase (MTase) of stoichiometry M2S1.  The fully active holoenzyme has a stoichiometry of R2M2S, but the subassembly complex (R1-complex lacks one of the HsdR subunits thus preventing restriction activity.