Ufr/s Variation in Escherichia Coli K12: a Reversible Double-mutation or Alternate Chromosome Expression in Non-complementing Diploids?

A novel form of bacterial variation found in an FhuA- mutant of Escherichia coli K12 was characterized by the alternation of (1) simultaneous resistance to lipopolysaccharide-specific phage U3 and to FhuA-specific agents (Ufr phenotype); and (2) a return to the sensitivity pattern of the initial strain (Ufs). In Ufr cells, loss of the U3 receptor permitted C21 adsorption without modifying the sensitivity to other tested phages or colicins. Genetic analysis revealed that Ufr variants were altered at two distinct loci. Ufr bacteria, though derived from a strain F- devoid of classical gene transfer mechanisms, were transiently able to promote mating between themselves and, to some extent, with other bacteria, including Rec-. Heterogenic matings resulted in the formation of persistent heterozygotes segregating Ufr- and Ufs-like bacteria. Pedigree analysis and subcloning of heterozygotic isolates indicate that they were diploids, as was the initial Ufr strain. Functional genetic complementation between these two genomes was only transient and the alternative forms were likely to result from the expression of a single chromosome of the heterozygotes. Mutation occurred in either form without causing any change in the alternative form.