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The assembly of plaque-forming particles in cell-free extracts of induced lambda lysogens was observed two ways. (i) DNA isolated from a X-related phage, 434 for example, is added to an extract of an induced X lysogen, and plaque-formers with the genotype of the added DNA are detected. (ii) One extract from an induced X lysogen that carries an amber mutation in one of the head genes (A, B, C, D, or E) is mixed with one carrying an amber mutation in a different head gene; an increase in the number of X plaque-formers is found over that in either extract alone. These plaque-forming particles have the properties of normal phage particles. They are resistant to DNase, although DNase added to an extract before addition of DNA prevents their appearance; they have a sensitivity to neutralizing antibody and a specificity of adsorption to bacteria characteristic of the source of the extract, but they have the genotype of the added DNA; and they have about the same bouyant density as phage particles.Mutants in genes B, C, or D can donate DNA to the phage formed by complementation between extracts of different mutants, but mutants in genes A or E cannot. Complementation occurs between a pair of extracts only if one (or both) is a DNA donor. This observation suggests a tentative pathway for head assembly: that the products of genes A and E act before those of B, C, and D.

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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 t w o 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. most abundant head protein, is added. These new observations are combined with published data to develop a comprehensive view of h head assembly.Protein A and DNA first react with the protein shells, then D protein, the second A macromolecular assembly process becomes irreversible if the covalent structure of the constituent subunits is altered after assembly has taken place. The head o f bacteriophage lambda contains DNA and several proteins. During head assembly, the primary structures of three of the proteins and the DNA are altered by specific processing reactions. The purpose of this article is t o show how processing is a necessary part of assembly.Let us first consider the final state of the structure we wish t o build. The h head is a shell of protein covering a single molecule of DNA. Figure 1 shows these structures after negative staining. Some particles have lost their DNA as evidenced by entrance of stain into the head. The protein shell is 55 nm in diameter and since it encloses a DNA molecule 15,000 n m long, the DNA must be folded back and forth some 250 times. The surface of the head is covered by a regular array of bumps and the pattern of ''fingers'' described b y Williams and Richards (1) is evident o n some of the particles. 3180 1974 Alan R. Liss, Inc.,

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