A bacterial position effect: when the F factor in E. coli K12 is integrated in cis to a chromosomal gene that is flanked by IS1 repeats the elements are activated so that amplification and other regulatory changes that affect the gene can occur.

In Escherichia coli K12 the argF gene is located within Tn2901, a genomic unit of approx. 12 kb that is flanked by IS1 elements in direct repeat. When strains in which the F factor is integrated in cis to Tn2901 are subjected to the appropriate selection, regulatory changes that result in over-production of the argF-encoded ornithine transcarbamylase occur at relatively high frequencies. When amplification occurs, the F factor is required only for the initial exchange between the IS1 elements, homologous recombination between tandem repeats of Tn2901 being independent of the F status of the cell. While amplification of Tn2901 is frequent in some Hfr strains, in others regulatory changes that do not involve amplification are predominant. The transfer region of the F factor does not contribute to the position effect.