Delayed expression of in vivo restriction activity following conjugal transfer of Escherichia coli hsdK (restriction-modification) genes

Following conjugal transfer of the hsdK genes (hsdRK, hsdMK, and hsdSK) of Escherichia coli K-12, restriction activity was first detected only after approximately 15 generations, whereas modification activity was observed immediately. This sequential expression explains the establishment of hsdK genes in a nonmodified host and suggests regulation of restriction activity after conjugal transfer.