Processing and assembly of the head of bacteriophage lambda

When phage DNA is added to an extract of an induced lambda lysogen, complete phage particles are made that contain the added DNA. The DNA substrate for packaging is a covalently joined polymer of several phage units. Unjoined units must first be joined by DNA ligase in the extract. Therefore DNA cutting is a necessary part of the DNA packaging reaction. The protein product of gene A, called A protein, behaves like the enzyme that cuts DNA and is a necessary component of the extract. Three of the head proteins preassemble into a spherical shell that subsequently combines with DNA. These shells are made of E protein, the major protein of a finished head, and they can be the sole source of that protein. They also contain a few molecules of two processed proteins, fused C–E and cleaved B. The processing may be essential for assembly because other shells that contain C protein not fused and B protein uncleaved are less than 1% as active. Protein A and DNA first react with the protein shells, then D protein, the second most abundant head protein, is added. These new observations are combined with published data to develop a comprehensive view of λ head assembly.