Overproduction of the hsd subunits leads to the loss of temperature-sensitive restriction and modification phenotype

The geneshsdM andhsdS for M.EcoKI modification methyltrasferase and the complete set ofhsdR,hsdM andhsdS genes coding for R.EcoKI restriction endonuclease, both with and without a temperature-sensitive (ts) mutation inhsdS gene, were cloned in pBR322 plasmid and introduced intoE. coli C (a strain without a natural restriction-modification (R-M) system). The strains producing only the methyltransferase, or together with the endonuclease, were thus obtained. ThehsdS _ts-1 mutation, mapped previously in the distal variable region of thehsdS gene with C^{1 245}-T transition has no effect on the R-M phenotype expressed from cloned genes in bacteria grown at 42°C. In clones transformed with the wholehsd region an alleviation of R-M functions was observed immediately after the transformation, but after subculture the transformants expressed the wild-type R-M phenotype irrespective of whether the wild-type or the mutanthsdS allele was present in the hybrid plasmid. Simultaneous overproduction of HsdS and HsdM subunits impairs the ts effect of thehsdS _ts-1 mutation on restriction and modification.