Although the two F' elements, F57 (carrying his(+)) and KLF1 (carrying leu(+)), cannot normally coexist in the same cell, crosses between RecA(-) strains of Escherichia coli carrying these two elements gave rise, at a low frequency, to progeny carrying both the his(+) and leu(+) markers in an extrachromosomal state. Genetic studies showed that the his(+) and leu(+) markers were now linked, and centrifugation studies of F' DNA isolated from these strains confirmed the presence of a new species of DNA molecule of altered size. It is proposed that the his(+) and leu(+) genes in these strains are covalently linked on a single "fused" F' element.
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Willetts et al. (1972) studied this question.