Evidence that F plasmid transfer replication underlies apparent adaptive mutation. 
An Escherichia coli K12 strain, FC40, has been used extensively in the analysis 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 medium.  These adaptive mutations are a subset of spontaneous types and their formation depends on the RecABC functions.  Data presented here suggest that this phenomenon depends on transfer functions of the F factor.  Fertility inhibition eliminates RecA-dependent adaptive reversion.  Thus, "adaptive" revertants may form during replication from the transfer origin, whereas loci in the nonreplicating chromosome show little mutation.