Polyomavirus Enhancer Binding Proteins PEA1, PEA2, and PEA3: Functional Analysis by In Vitro Transcription

Abstract

The polyomavirus enhancer consists of functionally redundant DNA sub-elements. One such sub-element, element 2, comprises a region with contiguous binding sites, or motifs, for at least three nuclear factors, designated as PEA1, PEA2, and PEA3. Although little is known of PEA2, PEA1 is presumed to be a murine homolog of human transcription activator protein 1 (AP-1), and PEA3 has recently been shown to be encoded by a member of the Ets family of oncogenes. The contributions of each factor to enhancer function are not understood. A cell-free system was devised to assay the individual abilities of the DNA motifs recognized by PEA1, PEA2, and PEA3 to confer transcriptional activation upon a minimal promoter. The motifs were cloned and tested as monomers, as multiple tandem copies, and in paired combinations. The results of these in vitro studies indicate that the PEA1 motif behaves as a low affinity AP-1 binding site; that PEA1 and PEA3, but not PEA2, activate transcription; and that both the PEA1 and PEA3 motifs act synergistically. Band shift titration experiments demonstrated that neither PEA1 nor PEA3 bound to their DNA motifs co-operatively, indicating that synergistic activation of transcription by these factors is not due to cooperative binding. Finally, additional in vitro transcription experiments suggest that PEA1 and PEA3 may co-operate with each other to stimulate transcription. A current model proposes that the minimal sub-units of enhancer structure are small (8-10 base pair) DNA motifs, called enhansons, that act synergistically. I propose that the motifs for PEA1 and PEA3, but not PEA2, are enhansons of the polyomavirus enhancer.