Polyoma large T can activate middle T expression by a hit-and-run mechanism.
We studied the activation of polyoma middle T expression in revertant cells carrying transcriptionally inactive copies of the middle T (pmt) oncogene. Introduction of polyoma large T with neo into a rat cell line containing multiple copies of pmt stably integrated into the genome corrected the transformation defect in some of the transfected cells by activating the resident pmt gene. However, once the cells were transformed, continuous expression of the large T protein was not required for the maintenance of pmt expression and hence the maintenance of the transformed state. Transformants arising spontaneously as well as those induced by large T exhibited frequent rearrangements of the pmt inserts. Our results suggest that large T activated pmt expression by a hit-and-run mechanism involving recombination of sequences in the viral insert.