Nitrosoguanidine assay of episomal integration in Escherichia coli.

Mutations affecting utilization of lactose and resistance to the male-specific phages f1, f2, and Qbeta tend to occur simultaneously more often than expected by chance in Hfr strains whose origin of transfer is close to the genes for lactose utilization, but not in F(+) strains. Strains derived from the Hfr, but exhibiting poor ability to transfer early chromosomal genes, may or may not show this comutation phenomenon. These results support the concept that the F factor is integrated into the Hfr chromosome during vegetative growth, but is autonomous in the F(+) strains and could serve as an assay for episomal localization.