In Vitro Assembly of Bacteriophage Lambda Heads

Abstract

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