Class I genes of the major histocompatibility complex (MHC) appear to be activated in mouse cells transformed by the DNA tumour virus simian virus 40 (SV40). Conversely, suppression of MHC class I genes has been reported in adenovirus-12-transformed baby kidney rat cells. We have now investigated the expression of genes encoded by the rat MHC locus in rat fibroblasts cells transformed by polyoma virus (Py). Using a mouse genomic H-2 clone as a probe in Northern transfer hybridization analysis, we have observed a high level of expression of rat MHC class I messenger RNA in all the transformed rat cell lines analysed. The class I 1.6-kilobase (kb) mRNA activated in Py-transformed rat cells appears to contain an Alu-like type II repeat element, as the same 1.6-kb mRNA is detected using either the H-2 class I sequence or a repetitive Alu-like type II element as a probe. High levels of heterogeneous poly(A)+ transcripts of 0.5-0.8 kb are also observed in Py-transformed rat cells using probes containing an Alu-like type II repetitive element.
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