Isolation of genes specifically expressed in flat revertant cells derived from activated ras-transformed NIH 3T3 cells by treatment with azatyrosine.
We previously reported that mouse NIH 3T3 cells transformed by transfection of activated human c-Ha-ras become apparently normal upon treatment with the antibiotic azatyrosine. The revertant cells maintain their normal phenotype during prolonged culture in the absence of azatyrosine, although activated p21ras is still expressed. The normal phenotype induced by azatyrosine could be due to activation of expression of some cellular gene(s) in the cells that results in suppression of ras function. To identify the genes with increased expression in the revertant cells, we adopted differential screening of recombinants from a phage cDNA library made from mRNA of the revertant cells, hybridized with 32P-labeled cDNAs made from mRNAs of the ras-transformed NIH 3T3 cells and the revertant cells. Two clones thus isolated were found to be almost identical to the ras recision gene (rrg), which was identified as a tumor-suppressor gene by Contente et al. [Contente, S., Kenyon, K., Rimoldi, D. & Friedman, R. M. (1990) Science 249, 796-798]. Other genes identified were the collagen type III and rhoB genes. Approximately half the clones were found to contain a sequence corresponding to that of the murine retrovirus-like intracisternal A particle. We speculate that azatyrosine activates several cellular genes in the ras-transformed cells and that some of these genes, including rrg, act cooperatively to counteract ras function, resulting in reversion of the ras-transformed cells to the normal phenotype.