Attachment of wild-typeFfactor to a specific chromosomal region in a variant strain ofEscherichia coliK12: the phenomenon of episomic alternation

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Abstract
A variant strain ofEscherichia coliK12 (♂3) was obtained after ultra-violet irradiation. This strain differs from wild type in the possession of a preferred chromosomal site for theFfactor.When wild-type females are infected with theFfactor they are converted toF+ males which carry theFfactor in the infectious state. TheseF+ males (standard) change with low frequency toHfrmales which carry theFfactor on the chromosome. When the variant female (♀3) is infected with theFfactor it is converted to anF+ male (state II) which changes with high frequency to a specificHfrmale (state I) which carries theFfactor at the preferred site (Hfr3).Male strains carrying the preferred site alternate between the specificHfrstate and theF+ state. The mechanism of this alternation is the back-and-forth shift of theFfactor between its integrated state of multiplication at theHfr3locus and its autonomous state of multiplication. This process is termedepisomic alternation.Episomic alternation offers a physical model for regular changes in state of the gene.TheHfr3locus 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 theFfactor is present atHfr3, and state II in which it is not.

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