Packaging

gal gene att L Lambda DNA

Lambda DNA

Lambda DNA attH Wogene

FIGURE 18.10 Packaging of Host DNA During Transduction by Lambda

When lambda phage enters its lytic cycle and makes phage particles, it usually packages the lambda DNA between the attL and attR sites. Occasionally, a mistake will occur, and part of the bacterial chromosome DNA will be packaged. Since lambda DNA normally integrates between the gal and bio genes of the E. coli chromosome, the defective lambda particles will most likely contain one or other of these genes.

DNA approximately the same length as the lambda genome in order for it to fit into the phage head. Consequently, specialized transducing particles arise only at extremely low frequency. However, once a lambda transducing phage has been created, it may re-integrate its DNA into the chromosome of another host cell. This may occur either in the attl site or into the chromosomal copy of the gene (usually gal or bio) carried by the lambda transducing phage. Inducing this defective prophage DNA will give a second generation of transducing phage particles at a much higher frequency.

The properties of lambda transducing phages depend on which lambda genes were lost in exchange for chromosomal DNA. The Idgal transducing phages lack lambda genes needed for making head and tail components and are therefore "defective" (hence the "d" in Idgal), but instead, the virus contains the E. coli gal gene. Defective phage may be grown together with a wild-type lambda as a helper phage to provide the missing functions. In the case of Idgal, helper phage would make the head and tail components. Another example, the Ipbio transducing phages lack the Lambda int gene, which integrates the phage DNA into the attl site, and instead contains the bio gene from E. coli. Since the phage cannot integrate, Ipbio must enter the lytic phase, and therefore, are obligate plaque formers (hence the "p" in Ipbio). If wild-type helper phage are added, the int function is restored, and the phage can form lysogens. Cloning vectors derived from lambda are widely used in genetic engineering (see Ch. 22). Since the cloned DNA replaces many of the lambda genes, such vectors need to be grown in the presence of helper phages.

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