THE EFFECT OF RECA MUTATION ON THE EXPRESSION OF ECOKI AND ECOR124I HSD GENES CLONED IN A MULTICOPY PLASMID

Type I restriction-modification (R-M) endonucleases are composed of th ree subunits - HsdR, required for restriction, and HsdM and HsdS which can produce a separate DNA methyltransferase. The HsdS subunit is req uired for DNA recognition. In this paper we describe the effect of clo ned EcoKI and EcoR124I hsd genes on the resulting R-M phenotype. The v ariability in the expression of the wild type (wt) restriction phenoty pe after cloning of the wt hsd genes in a multicopy plasmid in Escheri chia coli recA(+) background suggests that the increased production of the restriction endonuclease from pBR322 is detrimental to the cell a nd this leads to the deletion of the cloned hsd genes from the hybrid plasmid and/or inactivation of the enzyme. The effect of a mutation in E. coli reel gene on the expression of R-M phenotype is described and discussed in relation to the role of the cell surface and the localiz ation of the restriction endonuclease in the cell.