TRANSCRIPTION ACTIVATION MEDIATED BY CHROMOSOMAL INVERSION IN RAT-CELLS

To investigate the relationship between the local configuration of a g ene and its level of expression, we constructed a rat cell line, Hy5, carrying a mutant polyomavirus middle T oncogene (pmt) whose overexpre ssion converted the cells to the transformed state. The structure of t he transgene was such that pmt was able to undergo chromosomal inversi on at a relatively high rate by a cross-over in flanking pBR322 sequen ces. Hy5 cells became spontaneously transformed at a rate of 10(-5) pe r cell generation and all of the transformants analysed had sustained pmt inversion. CpG sequences were partially methylated in the Hy5 inse rt but appeared demethylated in transformants. In two subclones derive d from untransformed Hy5 cells, the pmt insert was densely methylated, transcriptionally inactive and unable to undergo homologous recombina tion. Our results suggest that DNA repair associated with recombinatio nal events leads to a heritable hypomethylation of the locus which is responsible for its activation.