During the assembly of bacteriophage λ heads, a head-like, DNA-free structure called petite λ, is first constructed. Into this, λ DNA is then packaged. In this paper we examine early interactions between λ DNA and petite λ in a cell-free system. The two major findings of this paper are: (1) when seen through the electron microscope, an early petite λ-λ DNA complex appears with the circular petite λ having the DNA crossing through its center. These resemble a bead on a string or the Greek letter φ (hence they are called φ structures). The λ A protein is required in the formation of φ structures. Also, φ structures can be found in bacteria infected with phage λ. (2) The polyamine putrescine is required for phage head assembly. An earlier reported requirement for spermidine can be replaced by the addition of putrescine. Polyamine is required in the DNA packaging reaction after the packaging has begun.
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