The Type I Restriction-Modification (R-M) enzyme EcoR124I is unusual in that the final step in the subunit assembly pathway involves a weak interaction between the motor subunit (HsdR) and an assembly intermediate R1-complex. This R1complex has been shown to have motor activity, but is unable to cleave DNA. We have investigated the effect of cofactor binding on the stability of the R-M enzyme and using a predictive structural model for the motor subunit we have investigated mutations that affect subunit assembly. Various data show that the N-terminus of HsdR is flexible and that the cysteine at position 4 is ‘available’ for cross linking, attachment of fluorophores etc. This suggests a mechanism for surface attachment of the molecular motor.