In vitro packaging of mature phage DNA by Salmonella phage P22

Mature, headful-sized DNA extracted from the Salmonella phages P22 and L, and P22/L-hybrid phages can be encapsulated in vitro by means of a packaging system for exogenous DNA. The probability of packaging reaches about 10(-3) per headful-sized molecule. The absence of in vitro recombination was demonstrated, to eliminate the possibility that such a process had created concatemers. The endonucleolytic cut at the pac site, which initiates sequential packaging in vivo, does not occur with the mature DNA substrate in vitro. The position of pac on the molecule is not important but the pac-recognizing phage protein gp3 is indispensable for in vitro encapsulation.