Redundant homosexual F transfer facilitates selection-induced reversion of plasmid mutations

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Abstract
F plasmids use surface exclusion to prevent the redundant entry of additional F plasmids during active growth of the host cells. This mechanism is relaxed during stationary phase and nonlethal selections, allowing homosexual redundant plasmid transfer. Homosexual redundant transfer occurs in stationary-phase liquid cultures, within nongrowing populations on solid media, and on media lacking a carbon source. We examined the relationship between homosexual redundant transfer, which occurs between F+ hosts, and reversion of a plasmid-encoded lac mutant allele, lacI33omegalacZ. Sodium dodecyl sulfate (SDS) and mutations that prevent normal transfer to F- cells reduced redundant transfer and selection-induced reversion of the lacI33omegalacZ allele. A recA null mutation reduced redundant transfer and selection-induced reversion of the lacI33omegalacZ mutation. Conversely, a recD null mutation increased redundant transfer and selection-induced reversion of the lacI33omegalacZ allele. These results suggest an explanation for why SDS and these mutations affect reversion of the plasmid lacI33omegalacZ allele. However, a direct causal relationship between transfer and reversion remains to be established. These results suggest that Rec proteins play an active role in redundant transfer and/or that redundant transfer is regulated with the activation of recombination. Redundant homosexual plasmid transfer during a period of stress may represent a genetic response that facilitates evolution of plasmid-encoded functions through mutation, recombination, reassortment, and dissemination of genetic elements present in the populations.