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 the E. coli[pCP1005] strain at 4 degrees 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 in E. coli strain transformed with the high-copy number plasmid pCP1005 carrying EcoR124I hsdR, M and S genes cloned, but not with strains carrying the natural conjugative plasmid R124. The effect of both treatments on the expression of EcoR124I phenotype in relation to the possible location of R.EcoR124I restriction endonuclease in E. coli is discussed.