Lambda Phage Assembly
The presence of a coat on phage lambda, and most other virus particles as well, can be broken down into three basic problems. How is the coat assembled? How is the nucleic acid placed inside the coat? How is the nucleic acid released from the virus particle into the appropriate cell? At present, partial answers are known to these questions, but it is not yet possible even to imagine the shapes of a set of folded proteins that would self-assemble into a structure the size of a phage, encapsidate DNA, and when properly triggered, release the DNA.
Not only is the structure of lambda phage dramatically different from a ribosome, but its assembly process also is notably different. First, as in the case of many larger viruses, one virus-encoded protein is used
Figure 21.10 Three schemesfor maturation of head subunits and the lambda DNA molecule into an assembled head with the DNA inside.
during assembly but is not present in the final particle. Second, a host protein participates enzymatically in the assembly process. Third, a number of proteins are cleaved or covalently joined during assembly.
An interesting problem involved in phage maturation is how the DNA is packaged in the head. There are three possible models for encapsida-tion of the DNA in the head (Fig. 21.10). The first is condensation of the DNA followed by assembly of the protein capsid around it. The second is a concerted condensation of DNA and capsid components, and the third is insertion of the DNA into a preformed capsid. Of these three possibilities, phage lambda matures by the third.