Assembly of biologically active proheads of bacteriophage lambda in vitro.

Bacteriophage lambda DNA can be packaged in vitro into preformed proheads to generate plaque-forming units. This complex set of reactions is initiated when lambda DNA is mixed with the product of the phage A gene, and proheads. Because proheads are an essential early reactant, the system has potential as an assay for the formation of biologically active proheads. When extracts of cells infected with certain lambda head mutants (for example, B--, C--, Nu3--, and E--) are used as the prohead donor, plaque-forming units are not produced. However, when extracts of E- - and Nu3- - infected cells are first reacted together the combination provides prohead-donor activity to the in vitro packaging system. In vitro assembled, biologically active proheads have the same sedimentation properties and electron micrsocopic appearance as "wild-type" proheads isolated from lambdaA-D- -infected cells. Centrifugation analysis shows that the Nu3- extract contributes gpE, the major capsid protein, to the reaction in the form of monomers or small polymers.