EVIDENCE THAT F-PLASMID TRANSFER REPLICATION UNDERLIES APPARENT ADAPTIVE MUTATION

Escherichia coli K12 strain, FC40, has been used extensively in the an alysis of adaptive mutability. This strain carries a revertible mutant lac allele on an F plasmid and accumulates Lac(+) (lactose utilizing) revertants, but not unselected mutants, when placed on selective medi um. These adaptive mutations are a subset of spontaneous types and the ir formation depends on the RecABC functions. Data presented here sugg est that this phenomenon depends on transfer functions of the F factor . Fertility inhibition eliminates RecA-dependent adaptive reversion. T hus, ''adaptive'' revertants may form during replication from the tran sfer origin, whereas loci in the nonreplicating chromosome show little mutation.