Increased rates of specific RNA polymerase III transcription precede overexpression of cellular oncogenes upon induction of transformation.

We have investigated a number of genes for possible overexpression upon induction of transformation, using mouse cells transformed with a temperature-sensitive mutant of simian virus 40 [SV40Ts(A)-transformed BALB-3T3, line A255], which express the transformed phenotype at 33 degrees C and suppress it as 39 degrees C. We examined both RNA polymerase II transcripts belonging to selected cellular oncogenes and RNA polymerase III transcripts of the B2 class. The selection was predicated on the basis of previous reports of an overexpression of these genes in transformed cell lines or tumors. Among the oncogenes, only cellular myc mRNA was found in increased amounts after transfer to the permissive temperature. However, nuclear run-on transcription assays indicated accelerated transcription rates for both c-myc and c-ras, with 2-3-fold and 4-5-fold increases at 2 and 6 hr post-induction, respectively. This acceleration was preceded by a 16-fold increase in the transcription rate of mouse B2 genes occurring within 30 min of the transfer to 33 degrees C. The sizeable and immediate burst in the rate of B2 transcription following the induction shift and the temporal order of activation of c-myc and c-ras are suggestive of a causal progression leading to full expression of the transformed phenotype.