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The decapsidation of several temperature-sensitive mutants of polyoma virus was compared with that of wild-type by infecting mouse embryo cells at low multiplicity simultaneously with 131 I-labeled mutant and 125 I-labeled wild-type virus. Analysis of the cytoplasm revealed that the noncomplementing mutant ts-3 virutally failed to give rise to the subviral particles which are characteristic of wild-type virus decapsidation. Correspondingly, little or no DNA from the infecting ts-3 particles was found in the cell nucleus. In contrast, the late mutants ts-10 and ts-1260 were decapsidated to a greater extent than wild-type virus. The results obtained with these three mutants can be accounted for on the basis of mutational changes in structural polypeptides, conferring either increased or decreased stability to the viral particle. A decapsidation pattern similar to that of wild-type virus was registered for the early mutant ts-a. These data support a model for Py decapsidation which we have proposed (1).

Mouse embryo cells that had been exposed to polyoma virus (Py) labeled in its protein and/or DNA moiety were fractionated at various times after infection. Sedimentation analysis of the cytoplasm in 1 M NaCl solution revealed the presence of a labeled viral DNA-protein complex, termed NP, which is absent from virus preparations used for infection. This complex has a sedimentation coefficient of about 135 S and a DNA/protein ratio higher than that of virus, reflecting primarily loss of histone-like polypeptides. Another newly recognized species, a DNA-free complex (DFC) sedimenting at about 185 S, was identified in the cytoplasm. It contains reduced amounts of histone-like polypeptides and little or no DNA, as compared with virus. Unlike the material sedimenting like “full” virus particles which was also found in the cytoplasm, neither NP or DFC was stable in 2.7 M CsCl. Histone-like polypeptides lost from the above-mentioned species were detected in the cytoplasm as slowly sedimenting material. Radioactive material sedimenting as free DNA in 1 M NaCl solution was recovered from the nucleus and, occasionally, from the cytoplasm. It never represented more than 1.5% of the labeled DNA molecules present in virus at the time of infection. We propose a scheme for the decapsidation of Py in the cell. As the viron meanders in the cytoplasm, histone-like polypeptides are lost and NP is generated. The ensuing step is transfer of DNA to the nucleus, with a simultaneous reorganization of the capsid proteins into DFC. The viral DNA in the nucleus presumably is instrumental in directing the infectious process.