Restoration of mutability in non-mutable Escherichia coli carrying different plasmids

N and I group plasmids, which increase methylmethane sulfonate (MMS) mutagenesis in lexA+ strains of E. coli WP2 may be divided into two classes: those restoring part of the mutability of lexA- stains (class I) and those leaving lexA- strains non-mutable (class II). Almost complete restoration of MMS mutability is obtained by class I plasmids in a partially suppressed lexA rnm strain, while clase II plasmids cause far fewer MMS revertants in this strain than in lexA+. A pair of class I and II plasmids in lexA- shows a synergistic effect on mutability. These two classes do not coincide with plasmid division into incompatibility groups.