The isolation of different classes of antibiotic-supersensitive outer membrane permeability mutants of has been described previously (Sukupolvi 1984, 704-712). One of these, the SS-A mutation, sensitizes the bacteria to gentian violet and to hydrophobic antibiotics. The phenotype of the SS-A mutant was restored to normal when a cloned fragment of the F plasmid, or the R plasmid R6-5, carrying the genes and was introduced on a multicopy plasmid. The introduction of a plasmid carrying only the gene showed that this gene was sufficient to restore the phenotype. Only clones with functioning (irrespective of copy number) restored the normal antibiotic-resistant phenotype in the SS-A mutant. An incompatibility test using a donor strain which carried transposon Tn in the 60 MDa plasmid of and a recipient in which Tn was placed close to the SS-A mutation indicated that the SS-A mutation was located in the 60 MDa virulence plasmid (previously called the cryptic plasmid) of The introduction of the large virulence plasmid carrying the SS-A mutant allele into wild-type or resulted in strains with a phenotype identical to that of the original SS-A mutant.


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