During morphogenesis in vivo, bacteriophage T7 packages and cuts to mature size an end-to-end concatemer of its nonpermuted, terminally repetitious, double-stranded, mature DNA. Efficient production (90-100%) and packaging (20-35%) of concatemers has also been demonstrated in extracts of T7-infected cells (in vitro) (Son, M., Hayes, S. J., and Serwer, P.  Virology 162, 38-46). By use of both this procedure of in vitro DNA packaging and in-gel hybridization to packaged DNA fractionated by agarose gel electrophoresis, the specificity of packaging in vitro is found to depend on the presence of T7 gene 6 exonuclease (p6). In the absence of p6 in vitro, no concatemerization is detected and packaging of DNA nonhomologous to T7 DNA (bacteriophage P22 DNA) is as efficient (0.05-1.10/6) as the packaging of monomeric T7 DNA. Addition of p6 in vitro both stimulates the concatemerization-packaging of T7 DNA and suppresses the packaging of P22 DNA. The packaging efficiency for concatemeric T7 DNA is 29-611 × higher than that for monomeric T7 DNA. Inhibition of the packaging of P22 DNA by p6 is correlated with the formation of single-stranded P22 DNA ends. These data are explained by the hypothesis that a DNA molecule with a single-stranded end is packaged less efficiently than the same DNA without the single-stranded end. Testing this hypothesis in vivo reveals that both p6 and gene 3 endonuclease contribute to suppressing the packaging of host DNA.
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