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We have placed a 225‐bp fragment from the 5′ end of the mouse rDNA transcription unit (from ‐169 to +56) in front of the SV40 tumor antigen coding sequence. After microinjection of this chimeric plasmid into nuclei of mouse L‐cells expression of SV40 large T antigen has been observed. The expression of T antigen was dependent on the correct orientation of the rDNA fragment relative to the T antigen‐coding region and was seen only in mouse cells. This indicates that the 225‐bp rDNA fragment contains the sequence information required for pre‐rRNA transcription and demonstrates for the first time that a protein‐coding gene can be transcribed and expressed under the control of an RNA polymerase I promoter.

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