Altered stability and integration frequency of a F' factor in RNA polymerase mutants of Escherichia coli.

A number of spontaneous rifampicin-resistant (Rifr) mutants were isolated from a strain of E. coli having a deletion in the lac proA proB region of the chromosome. The stability of a F'lac proA proB episome in these mutants was determined by their sensitivity to acridine orange curing and the frequency of spontaneous loss of episomes. The Rifr mutants can be divided into three classes based on their ability to maintain the F'lac pro episome. Class I mutants (25% of the total Rifr mutants) showed high degree of spontaneous episome loss and high sensitivity to acridine orange curing. Class II mutants (55% of the total Rifr mutants), like the parent strains, showed intermediate sensitivity to acridine orange curing. Class III mutants (21% of the total Rifr mutants) showed high resistance to acridine orange curing and low frequency of spontaneous episome loss. Three-fourths of the Class II mutants were found to be Hfr as shown by their lack of the F'lac pro DNA band on agarose gel together with their ability to mobilize chromosomal markers in mating. Representative Rifr mutants from each class were selected and the Rifr mutants from each class were selected and the Rifr mutations were mapped within the proB gene for the beta beta' operon by P1 transduction. These results indicate that RNA polymerase, or the beta subunit of RNA polymerase, plays an important role in maintaining the F' lac pro episome and in the integration of the F' lac pro episome where no extensive sequence homology is involved.