Construction and functional characterization of polyomavirus genomes that separately encode the three early proteins.

Modified polyomavirus genomes that individually encode the large and small T proteins were constructed by exchanging restriction endonuclease fragments between cDNA copies of the respective mRNAs and cloned genomic DNA. The efficacies of the new constructs, and that of the middle T protein gene described previously (R. Treisman , U. Novak, J. Favaloro , and R. Kamen , Nature [London] 292:595-600, 1981), were demonstrated with simian virus 40 (SV40)-polyomavirus recombinants in which part or all of the SV40 late region was replaced with the modified polyomavirus early genes. Each of the three recombinant viruses induced the synthesis of only the expected polyomavirus early protein in infected CV-1 cells. The rates of synthesis of large, middle, and small T proteins were ca. 1.5, 4.0, and 9.0 times the rate of synthesis of SV40 large T protein, respectively. The deletion of introns had no detrimental effect on mRNA biogenesis. Indeed, a further polyomavirus-SV40 recombinant, containing wild-type polyomavirus early region DNA, expressed an aberrant 58,000-dalton form of the middle T protein which we believe to result from utilization of a cryptic splice site. Immunofluorescence studied with monkey cells infected by the recombinant viruses allowed us to determine the cellular locations of the polyomavirus early proteins. Overproduction of the middle T protein did not result in a corresponding overproduction of the middle T protein-associated tyrosine phosphokinase activity.