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

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Abstract
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.