REDUNDANT HOMOSEXUAL F-TRANSFER FACILITATES SELECTION-INDUCED REVERSION OF PLASMID MUTATIONS

F plasmids use surface exclusion to prevent the redundant entry of add itional F plasmids during active growth of the host cells. This mechan ism is relaxed during stationary phase and nonlethal selections, allow ing homosexual redundant plasmid transfer, Homosexual redundant transf er occurs in stationary-phase liquid cultures, within nongrowing popul ations on solid media, and on media lacking a carbon source, We examin ed the relationship between homosexual redundant transfer, which occur s between F+ hosts, and reversion of a plasmid-encoded lac mutant alle le, lacI33 Ohm lacZ. Sodium dodecyl sulfate (SDS) and mutations that p revent normal transfer to F- cells reduced redundant transfer and sele ction-induced reversion of the lacI33 Ohm lacZ allele, A recA null mut ation reduced redundant transfer and selection-induced reversion of th e lacI33 Ohm lacZ mutation, Conversely, a recD null mutation increased redundant transfer and selection-induced reversion of the lacI33 Ohm lacZ allele, These results suggest an explanation for why SDS and thes e mutations affect reversion of the plasmid lacI33 Ohm lacZ allele, Ho wever, a direct causal relationship between transfer and reversion rem ains to be established, These results suggest that Rec proteins play a n active role in redundant transfer and/or that redundant transfer is regulated with the activation of recombination, Redundant homosexual p lasmid transfer during a period of stress may represent a genetic resp onse that facilitates evolution of plasmid-encoded functions through m utation, recombination, reassortment, and dissemination of genetic ele ments present in the populations.