The EcoR124 and EcoR124/3 restriction and modification systems: cloning the genes.

Abstract The Esherichia coli plasmid R124 codes for a type I restriction and modification system Eco R124 and carries genetic information, most probably in the form of a “silent copy,” for the expression of a different R-M specificity R124/3. Characteristic DNA rearrangements have been shown to accompany the switch in specificity from R124 to R124/3 and vice versa. We have cloned a 14.2-kb Hind III fragment from R124 and shown that it contains the hsd R, hsd M, and hsd S genes which code for the Eco R124 R-M system. An equivalent fragment from the plasmid R124/3 following the switch in R-M specificity has also been cloned and shown to contain the genes coding for the Eco R 124/3 R-M system. These fragments, however, lack a component present on the wild-type plasmid essential for the switch in specificity. Restriction fragment maps and preliminary heteroduplex analysis indicate the near identity of the genes that encode the two different DNA recognition specificities. Transposon mutagenesis was used to locate the positions of the hsd R, hsd M, and hsd S genes on the cloned fragments in conjunction with complementation tests for gene function. Indirect evidence indicates that hsd R is expressed from its own promoter and that hsd M and hsd S are expressed from a single promoter, unidirectionally.