Regulation of the expression of the SV40 T-antigen coding gene under the control of an rDNA promoter.

We have constructed a hybrid gene in which the SV40 T-antigen coding gene is driven by a mouse rDNA promoter and we have compared its expression to that of an SV40 T-antigen coding gene under the control of its own promoter. The comparison has been carried out in microinjected cells, in transfected cells, and in stable cell lines carrying the respective T-antigen coding genes in an integrated form. These cell lines were derived from ts AF8 cells, a mutant which is temperature sensitive for RNA polymerase II activity. The hybrid gene clearly expresses T-antigen, albeit less efficiently than when the T antigen coding gene is under the control of the SV40-promoter. We also show that the expression of T-antigen by the hybrid gene is 50% inhibited by an antibody against RNA polymerase I. In tsAF8 cells carrying the hybrid gene, T-antigen is still expressed at the restrictive temperature (where RNA polymerase II is inactive) at a level again about 50% of controls. However, our findings also confirm those of Smale and Tjian (Mol. Cell. Biol. 5:352, 1985) that such hybrid genes are in part transcribed by RNA polymerase II and generate abnormal transcripts.