The middle T proteins of high and low tumor strains of polyomavirus function equivalently in tumor induction.

The PTA strain of polyomavirus induces a variety of epithelial as well as mesenchymal tumors at high frequencies, while the RA strain induces only rare mesenchymal tumors following inoculation into newborn mice. DNA sequence analysis has revealed one amino acid difference between the middle T (mT) proteins encoded by these two virus strains. To test for possible biological differences between the mT proteins we constructed a recombinant virus carrying mT-coding sequences of RA on a PTA background. The tumor profile induced by this recombinant is like that of PTA, demonstrating that the transforming protein of the low tumor strain is competent to induce a high tumor profile. We conclude that a structural determinant(s) outside of mT in the PTA virus strain is important in induction of a broad spectrum of tumors and particularly those of epithelial origin.