sitesattBandattP, respectively, to yield product sites known asattLandattR. These were
explained in detail in Chapter 7 and are the basis of the Gateway cloning system. The
reaction can be excision, inversion, or integration, and is not reversible unless an additional
protein, an excisionase, is provided. With some members of this group, the integrase can
catalyze recombination without the need of a host protein; examples of these recombination
systems includefC31,fBT1, and Bxb1.
16.2.1 Removal of DNA from Transgenic Plants or Plant Parts
The reciprocal exchange of strategically placed recombination sites offers a variety of appli-
cations. For instance, the specific deletion of DNA provides a means to remove transgenic
materials not needed in the commercial product, in the consumed portion of the commercial
crop, or in plant pollen. The site-specific insertion of DNA into pre-defined sites in the
genome could help minimize unwanted “position effects” caused by the random integration
of introduced DNA. The rearrangement of DNA could also be used to alter gene expression,
as in a genetic switch to turn on or off a large set of genes; and the specific recombination
Figure 16.1.Recombination between recombination sites (arrowheads) leading to (a) deletion
(excision of circular molecule 2,3 from molecule 1,2,3,4) or integration (insertion of molecule
2,3 into molecule 1,4); (b) inversion (of DNA segment 2,3 flanked by recombination sites of opposite
orientation); or (c) translocation (of DNA of different molecules). Some recombination systems use
recombination sites that differ in sequence generally known asattB,attP,attL, andattR, shown
here as BB^0 ,PP^0 ,BP^0 , and PB^0 , respectively. In these systems, recombination betweenattLand
attRrequires an excisionase protein in addition to an integrase protein.
16.2. SITE-SPECIFIC RECOMBINATION SYSTEMS TO PROVIDE INCREASED PRECISION 361