Several hypotheses—later supported—followed from Campbell's proposal.
1. One end of the integrated F factor would be the origin, where transfer of the Hfr chromosome begins. The terminus would be at the other end of F.
2. The orientation in which F is inserted would determine the order of entry of donor alleles. If the circle contains genes A, B, C, and D, then insertion between A and D would give the order ABCD or DCBA, depending on orientation. Check the different orientations of the insertions in Figure 5-12.
How is it possible for F to integrate at different sites? If F DNA had a region homologous to any of several regions on the bacterial chromosome, any one of these could act as a pairing region at which pairing could be followed by a crossover. These regions of homology are now known to be mainly segments of transposable elements called insertion sequences. For a full explanation of these, see Chapter 13.
The fertility factor thus exists in two states:
1. The plasmid state: as a free cytoplasmic element F is easily transferred to F~ recipients.
2. The integrated state: as a contiguous part of a circular chromosome F is transmitted only very late in conjugation.
The E. coli conjugation cycle is summarized in Figure 5-14.
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