Morphogenesis of phi X174: in vitro synthesis of infectious phage from purified viral components.

An in vitro system that synthesizes infectious phage phi X174 was developed. The synthesis depended on phi X174 supercoiled replicative form DNA, purified phi X174 gene A protein, gene C protein, gene J protein, prohead (phage head precursor composed of gene F, G, H, B, and D proteins), and uninfected host crude extract. The infectious phage synthesis was coupled with DNA synthesis. De novo initiation, elongation, and termination of phi X174 single-stranded DNA was observed. The phage synthesized in vitro cosedimented with in vivo phage in sucrose gradients and had the same buoyant density as in vivo phage in a CsCl gradient. Our results indicate that the in vitro system mimics the in vivo phi X174 assembly process.