A complementation analysis of the restriction and modification of DNA in Escherichia coli

Abstract

A complementation analysis of host-controlled modification and restriction of DNA by Escherichia coli has been carried out by examining the restriction and modification phenotypes of partial, permanent diploids containing various arrangements of wild type and mutant restriction and modification alleles. Intercistronic complementation was observed between three classes of restriction and modification mutants of E. coli B, indicating that at least three cistrons (the ram cistrons) are involved in the genetic control of the [restriction and modification of DNA. Mutations in one cistron (ramA) result in a loss of restriction activity but not in modification activity (r⁻m⁺). Mutations in a second cistron (ramC) result in a loss of restriction and modification activities (r⁻m⁻). Mutations in a third cistron result in a loss of modification activity and appear to be lethal unless accompanied by a mutation in the ramA or ramC cistrons. A fourth class of mutations, which are linked to the other ram cistrons and are expressed phenotypically as r⁻m⁻ mutants, are trans dominant to the wild-type ram alleles. It is not known if this latter class of mutants represents a fourth cistron of the ram locus. Complementation was observed between E. coli K12 and B ramA and ramC mutations and the host specificity of the restored restriction activity was dependent on an intact ramC cistron. However, complementation was not detected between the P1 and K12 or P1 and B ram alleles. A general model for the genetic control of the restriction and modification properties of E. coli strains and their episomes is presented.