While cellular transformation by small DNA tumour viruses usually involves targeting the product of the p53 tumour suppressor gene by a virally encoded protein, none of the three polyoma virus (Py) speci®ed T antigens have been observed to interact with p53. We show that primary mouse embryo ®broblasts and REF52 cells, which resemble primary cells in requiring co-operating oncogenes for transformation, cannot be transformed by the Py oncogene, middle T-antigen (PyMT), alone. These cells can be transformed by the complete Py early region, which encodes the Py large, middle and small Tantigens. We ®nd that PyMT can transform rodent cells lacking a functional p53 protein (p53 null mouse embryo ®broblasts and DN-REF52 cells which contain a dominant negative p53). In Py transformed REF52 cells (Py-REF52) there is no signi®cant accumulation of p53 protein, as opposed to SV40 transformed REF52 cells (SV-REF52) in which the amount of steady state p53 protein is elevated. However accumulation of p53 is observed following exposure of Py-REF52 cells to u.v. Treatment of Py-REF52 cells with X-rays results in a rapid increase in the levels of the p53-induced proteins p21/WAF1 and MDM2. In untransformed REF52 cells, X-irradiation causes p53 activation, which results in induction of both G1/S and G2/M blocks. In SV-REF52 and DN-REF52 cells, p53 abrogation results in the absence of both the G1/S and G2/M blocks. Only the absence of a G1/S block is observed in Py-REF52 cells exposed to X-irradiation. Together these results indicate that in contrast to most other DNA tumour viruses, Py does not appear to interefere with the DNA damage induced transactivation activities of the p53 protein but absence of a functional p53 protein can mediate transformation by the PyMT oncogene in the absence of other co-operating oncogenes. Possible modes of transformation by Py are discussed.

No abstract

No abstract

Temperature sensitive cells have been isolated from Syrian and Chinese hamster cells using a method based on selective detachment from a glass substrate. The Syrian hamster isolates occurred at a high frequency (about 1 in 10") and reverted rapidly; polyoma virus transformation conferred on cells the ability to grow, perhaps abnormally, in agar suspension. A slightly modified isolation technique was applied to Chinese hamster cultures and resulted in the isolation of' at least one mutant (from a starting population of 5 X 108 cells) with a spontaneous reversion rate of less than one in 6 X 10'. Treatment of the mutant with ethyl methane sulphonate induced reversion. It was concluded that selective detachment provided a useful method for the isolation of conditional lethal mutants of mammalian cells.

No abstract