Initiation of sequential packaging of bacteriophage P22 DNA

Abstract

Bacteriophage P22 is thought to package and cut its double-stranded DNA chromosome from concatemeric replicating DNA in a “processive,” sequential fashion. According to this model, during the initial event in such a series the packaging apparatus first recognizes a base sequence, called pac, on the concatemeric phage DNA, and then condenses DNA within the phage head unidirectionally from that point. The DNA is cut at or near the pac site before or during this condensation, and a second cut is made, which separates the packaged DNA from the unpackaged DNA, when the head is full. Subsequent packaging events on that concatemeric substrate molecule then proceed from the end generated by the first event, in the same direction along the concatemer as the first event. In this paper we present evidence that the cuts made at the pac site are in fact not exact, but occur predominantly in six regions, called end sites, located within a 120 base-pair portion of the P22 chromosome.