Directional transfer and mapping Different Hfr strains have the F factor integrated into the bacterial chromosome at different sites and in different orientations. Gene transfer always begins within F, and the orientation and position of F determine the direction and starting point of gene transfer. In 4 FIGURE 8.18a, strain Hfr1 has F integrated between leu and azi; the orientation of F at this site dictates that gene transfer will proceed in a counterclockwise direction around the circular chromosome. Genes from this strain will be transferred in the order of:

; leu-thr-thi-his-gal-lac-pro-azi

Strain Hfr5 has F integrated between the thi and his genes (< Figure 8.18b) and in the opposite orientation. Here gene transfer will proceed in a clockwise direction:

Although the starting point and direction of transfer may differ between two strains, the relative distance in time between any two pairs of genes is constant.

Notice that the order of gene transfer is not the same for different Hfr strains ( FIGURE 8.19a). For example, azi is transferred just after leu in strain HfrH, but long after leu in strain Hfr1. Aligning the sequences ( FIGURE 8.19b) shows that the two genes on either side of azi are always the same: leu and pro. That they are the same makes sense when one recognizes that the bacterial chromosome is circular and the starting point of transfer varies from strain to strain. These data provided the first evidence that the bacterial chromosome is circular ( FIGURE 8.19c).

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