The effect ofrecA mutation on the expression ofEcoKI andEcoR124Ihsd genes cloned in a multicopy plasmid

Type I restriction-modification (R-M) endonucleases are composed of three subunits—HsdR, required for restriction, and HsdM and HsdS which can produce a separate DNA methyltransferase. The HsdS subunit is required for DNA recognition. In this paper we describe the effect of clonedEcoKI andEcoR124Ihsd genes on the resulting R-M phenotype. The variability in the expression of the wild type (wt) restriction phenotype after cloning of the wthsd genes in a multicopy plasmid inEscherichia coli recA ⁺ background suggests that the increased production of the restriction endonuclease from pBR322 is detrimental to the cell and this leads to the deletion of the clonedhsd genes from the hybrid plasmid and/or inactivation of the enzyme. The effect of a mutation inE. coli recA gene on the expression of R-M phenotype is described and discussed in relation to the role of the cell surface and the localization of the restriction endonuclease in the cell.