Regulation and DNA synthesis
FIGURE 22.19 Lambda Replacement Vector
Since lambda phage is easy to grow and manipulate, the genome has been modified to accept foreign DNA inserts. The green region of the genome has genes that are non-essential for lambda growth and packaging. This region can be replaced with large inserts of foreign DNA (up to about 23 kb). When used with a helper phage, such modified lambdas provide useful cloning vectors.
themselves. To generate lysogens it is necessary to use a helper phage to provide the integration and recombination functions.
If foreign DNA is inserted into the middle of lambda, the result is a linear DNA molecule with two cohesive ends. To get such constructs into an E. coli host cell efficiently requires in vitro packaging (Fig. 22.20). In this technique, a mixture of lambda proteins is mixed with the recombinant lambda DNA in vitro to form phage particles. Infecting two separate E. coli cultures with two different defective lambda mutants generates the necessary lambda proteins. Each of the two mutants lacks an essential head protein and cannot form particles containing its own DNA. A mixture of the two lysates gives a full set of lambda proteins and when mixed with lambda DNA can generate infectious phage particles.
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