Analysis of the subunit assembly of the typeIC restriction-modification enzyme EcoR124I.

Type I restriction-modification (R-M) enzymes are composed of three different subunits, of which HsdS determines DNA specificity, HsdM is responsible for DNA methylation and HsdR is required for restriction. The HsdM and HsdS subunits can also form an independent DNA methyltransferase with a subunit stoichiometry of M2S1. We found that the purified Eco R124I R-M enzyme was a mixture of two species as detected by the presence of two differently migrating specific DNA-protein complexes in a gel retardation assay. An analysis of protein subunits isolated from the complexes indicated that the larger species had a stoichiometry of R2M2S1and the smaller species had a stoichiometry of R1M2S1. In vitro analysis of subunit assembly revealed that while binding of the first HsdR subunit to the M2S1complex was very tight, the second HsdR subunit was bound weakly and it dissociated from the R1M2S1complex with an apparent K d of approximately 2.4 x 10(-7) M. Functional assays have shown that only the R2M2S1complex is capable of DNA cleavage, however, the R1M2S1complex retains ATPase activity. The relevance of this situation is discussed in terms of the regulation of restriction activity in vivo upon conjugative transfer of a plasmid-born R-M system into an unmodified host cell.