RNA polymerase II transcription blocked by Escherichia coli lac repressor

A reversible block to RNA polymerase II transcriptional elongation has been created with a lac operator sequence in the intron of the SV40 large T-antigen gene. When this transcription unit is injected into rabbit kidney cells expressing Escherichia coli lac repressor, T-antigen expression is reduced. This effect is not observed in cells lacking repressor or in the absence of the operator, and it is reversed by an inducer of the lac operon, namely isopropyl thiogalactoside (IPTG). In an extract of HeLa nuclei supplemented with lac repressor, this and similar constructs give rise to shortened transcripts that map to the 5′ boundary of the repressor-operator complex. These shorter RNAs are also sensitive to IPTG induction. This model system shows that a protein-DNA complex can block the passage of RNA polymerase II, and offers some insight into the control of eukaryotic gene expression during transcription elongation, a phenomenon observed in a variety of systems.