Restriction endonucleases R.EcoKI and R.EcoR124I are probably located in different environments within the bacterial cell

We describe the phenomenon of a transient state of R124I restriction deficiency after long-term storage of theE. coli[pCP1005] strain at 4°C, or after growth of the culture in synthetic M9 medium with the nonmutagenic solvent dimethyl sulfoxide. The unusual high reversion from the R⁺ ₁₂₄ to the R^? ₁₂₄ phenotype was observed only inE. coli strain transformed with the high-copy number plasmid pCP1005 carryingECoR124IhsdR, M and S genes cloned, but not with strains carrying the natural conjugative plasmid R124. The effect of both treatments on the expression ofEcoR124I phenotype in relation to the possible location of R.EcoR124I restriction endonuclease inE. coli is discussed.