Regulation of the activity of the type IC EcoR124I restriction enzyme.

Restriction-modification (R-M) systems must regulate the expression of their genes so that the chromosomal genome is modified at all times by the methyltransferase to protect the host cell from the potential lethal action of the cognate restriction endonuclease. Since type I R-M systems can be transferred to non-modified Escherichia coli cells by conjugation or transformation without killing the recipient, they must have some means to regulate their restriction activity upon entering a new host cell to avoid restriction of unprotected host DNA and cell death. This is especially true for EcoR124I, a type IC family member, which is coded for by a conjugative plasmid. Control of EcoR124I restriction activity is most likely at the post-translational level as the transfer of the EcoR124I system into a recipient cell that already expressed the HsdR subunit of this system was not a lethal event. Additionally, the kinetics of restriction activity upon transfer of the genes coding for the EcoR124I RM system to a recipient cell are the same, irrespective of the modification state of the recipient cell or the presence or absence of the EcoR124I HsdR subunit in the new host cells. The mechanism controlling the restriction activity of a type IC R-M system upon transfer to a new host cell is different from that controlling the chromosomally coded type IA and IB R-M systems. The previously discovered hsdC mutant, which affects the establishment of the type IA system EcoKI, was shown to affect the establishment of the type IB system EcoAI, but to have no influence on EcoR124I.