A mutant of Escherichia coli permitting replication of two F factors.

Control mechanisms of replication of bacterial genetic elements are poorly understood at present. We have studied one such mechanism involving replication of the F factor in Escherichia coli. The F factor can replicate either autonomously in F(+) or F' strains, or as an integral part of the chromosome in Hfr strains. We have shown that presence of either an integrated F factor or a free F factor prevents replication of a second free F factor. Two integrated F factors can replicate in the same cell. The present experiments show that when a F'lac element was transferred by mating into an Hfr strain, it had to become integrated into the chromosome in order to persist. With a recombination-deficient (recA) Hfr strain as recipient, the frequency of F'lac integration was greatly reduced. This permitted us to isolate a mutant Hfr strain in which the F'lac element was able to replicate autonomously. The mutation has most likely occurred in the integrated F factor itself. Availability of this new recA mutant Hfr strain facilitates genetic analysis of the F factor, since in recA(+) Hfr strains frequent integration of a free F factor into the chromosome obscures recognition of complementation and recombination between two free F factors.