A variant strain of Escherichia coli K12 (♂3) was obtained after ultra-violet irradiation. This strain differs from wild type in the possession of a preferred chromosomal site for the F factor.
When wild-type females are infected with the F factor they are converted to F + males which carry the F factor in the infectious state. These F + males (standard) change with low frequency to Hfr males which carry the F factor on the chromosome. When the variant female (♀3) is infected with the F factor it is converted to an F + male (state II) which changes with high frequency to a specific Hfr male (state I) which carries the F factor at the preferred site (Hfr3).
Male strains carrying the preferred site alternate between the specific Hfr state and the F + state. The mechanism of this alternation is the back-and-forth shift of the F factor between its integrated state of multiplication at the Hfr3 locus and its autonomous state of multiplication. This process is termed episomic alternation.
Episomic alternation offers a physical model for regular changes in state of the gene.
The Hfr3 locus has two allelic forms, high rate of change and standard rate of change. The allele for high rate of change may be considered as a genetic system which occurs in two possible states, state I in which the F factor is present at Hfr3, and state II in which it is not.