GENETIC INSTABILITY WITH EPISOME-MEDIATED TRANSFER IN ESCHERICHIA COLI

Abstract

Hospital strains of Escherichia coli have been simultaneously screened for an increased mutation rate to streptomycin (Sm) resistance and to auxotrophy. Two strains were found, with an increased mutation rate to Sm, but with no more than normal mutation rates in other loci tested. These loci included threonine, histidine, methionine, isoleucine-valine, resistance to tetracycline, and resistance to chloramphenicol. The genetic determinant of the new type of instability of the Sm locus has been studied. In contrast to the Treffers mutator gene, the new mutator character was not found to follow the pattern of a chromosomial factor in genetic cross. The character was found to possess the properties generally ascribed to genetic determinants of an episome nature: (i) a spontaneous loss of the character, enhanced by treatment with ultraviolet irradiation and acridine dyes; (ii) a loss which apparently is permanent; and (iii) a transfer to a mutator-negative cell by infection, both to a homologous mutator-negative mutant and to a K-12 strain. This infection is favored by the presence of the F factor in the donor.