Isolation and characterization of polyoma virus mutants able to develop in embryonal carcinoma cells.

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Abstract
Embryonal carcinoma (EC) mouse cells were resistant to infection by retroviruses and small oncogenic DNA viruses, including SV-40 and polyoma. When allowed to differentiate, in vitro or in vivo, EC cells become as susceptible to these viruses as differentiated mouse cell lines are. To study the relationships between differentiation of EC cells and viral expression, several polyoma mutants that can express early and late functions in undifferentiated EC cells were isolated and characterized. These mutants, which arose spontaneously during high-multiplicity infection of PCD3 cells (a differentiated fibroblast-like cell line derived from PCC3 EC cells), were selected on PCC4 cells (undifferentiated EC cells) and twice plaque purified. Restriction enzyme analysis of the DNA from several mutants showed that they all exhibit an additional sequence located in the Pvu II endonuclease fragment 4, close to the junction between Hpa II endonuclease fragments 3 and 5. The size of the insertion varies from 10-50 base pairs. The biological properties, including oncogenicity, transforming ability, host range and burst size of the mutants so far analyzed are similar to those of wild-type virus.