E. Lenk, S. Casjens, J. Weeks and J. King (1975) Virology, 68, 182-199

Intracellular visualization of precursor capsids in phage P22 mutant infected cells


The morphogenesis of the double-stranded DNA Salmonella phage P22 has been studied by electron microscopy of sections of wild type and mutant-infected cells. Previous work had shown that the precrusor capsid structure that encapsidates DNA is a complex of two major protein species, the gene 5 coat protein and the gene 8 scaffolding protein. Scaffolding protein exits from the precursor capsid in coupling with DNA encapsidation and then recycles.

No organized structures were seen in cells infected with amber mutants of the coat protein gene. Cells infected with an amber mutant of the scaffolding gene contained small numbers of aberrant perticles, including empty petit capsids and giant nested or spiral shell structures.

In cells infected with mutants blocked in DNA encapsidation (genes 1, 2, and 3) precursor capsids (proheads) accumulate amid the vegetative DNA and not along the membrane, as in phage T4. The proheads appear as double-shell structures about the size of mature phage, with the inner cell diameter about two-thirds the dimensions of the outer shell.

Mature phage and defective particles containing DNA form paracrystalline arrays within infected cells. Empty capsids, lacking both DNA and the iner shell of proheads, appear within the paracrystalline arrays of filled heads in cells infected with mutants blocked in head completion (genes 10 and 26). These empty capsids are presumably derived from filled but incomplete heads that have lost their DNA intracellularly.

Use of temperature-sensitive mutants blocked in the encapsidation steps allowed visualization of the first filled heads upon shift to permissive temperature. These particles tended to appear at the edge of the DNA pool. Partially filled particles with dense central cores often were seen associated with the growing paracrystalline arrays, and they probably represented intermediates in encapsidation.

These experiments, in conjunction wih others, suggest that the scaffolding protein, with functions in prohead assembly and perhaps in DNA encapsidation, is organized into an inner shell within the precursor capsid.

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