Mapping and cloning of Corynebacterium diphtheriae plasmid pNG2 and characterization of its relatedness to plasmids from skin coryneforms

The relationship of plasmid pNG2, isolated from an erythromycin-resistant strain of Corynebacterium diphtheriae, to plasmids isolated from skin coryneforms was examined. The extent of homology between plasmids from erythromycin-resistant and -susceptible skin coryneforms and pNG2 varied, but in aggregate homology was observed with all six BstEII fragments of pNG2. The data support the hypothesis that pNG2 originated in skin coryneforms. Intact plasmid pNG2 and some of its restriction fragments were cloned into Escherichia coli JM109. The erythromycin resistance phenotype was expressed in clones carrying intact pNG2 as well as in some of its fragments and appeared to depend on a C. diphtheriae promoter for expression. A 2.5-megadalton EcoRI fragment, the smallest expressing resistance, contained the 1.2-megadalton region of pNG2 which is deleted when the erythromycin-resistant strain of C. diphtheriae reverts spontaneously to the susceptible state.