In vitro assembly of bacteriophage Lambda heads.

The assembly of plaque-forming particles in cell-free extracts of induced lambda lysogens was observed two ways. (i) DNA isolated from a lambda-related phage, 434 for example, is added to an extract of an induced lambda lysogen, and plaque-formers with the genotype of the added DNA are detected. (ii) One extract from an induced lambda 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 lambda 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.