system makes use of the Cre-loxPsite-specific recombination system to transfer DNA frag-
ments from a reference plasmid, known as apUNIplasmid, to a recipient vector, known as
pHOST, to produce an expression vector. Both the pUNI and the pHOST vectors contain a
loxP site that permit site-specific recombination and the production of a cointegrate
(Fig. 7.16).
Figure 7.15.Positive selection of expression clones is provided by a chloramphenicol resistance
(CAT) gene in the “acceptor” vector. This gene can be expressed only when it is transferred from
the donor clone into the acceptor vector, where it is positioned adjacent to a prokaryotic promoter.
In this way, onlyE. colitransformed by acceptor vectors containing an insert will survive on
chloramphenicol-containing media. In this hypothetical example, a constitutive and ubiquitous
plant promoter present on the acceptor vector allows expression of the cDNA only when the fragment
of interest flanked byloxPsites is transferred to a singleloxPsite in the “acceptor” vector.
176 RECOMBINANT DNA, VECTOR DESIGN, AND CONSTRUCTION