Analysis of transcription factors binding to the duplicated PEA1 and PEA3 sites that are required for polyomavirus mutant expression in PCC4 embryonic carcinoma cells.

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Embryonic carcinoma (EC) cell lines, representative of early embryonic undifferentiated cells, are nonpermissive for polyomavirus (PyV) infection as a result of a blockade of viral DNA early transcription and replication. All enhancers of PyV mutants (Py EC-PCC4), selected for the ability to grow on PCC4 EC cells, display a duplication of PEA1 and PEA3 binding sites (sites 1 and 3). However, the Py EC-PCC4 rearrangement is complex and results in variable mutant enhancer activities. We demonstrate here that duplication of sites 1 and 3 is absolutely required for a cooperative cis activation of early Py EC-PCC4 mutant transcription in PCC4 EC cells. In addition, we detect in PCC4 EC cells significant amounts of site 1- and 3-binding proteins, which we characterize as related to the Fos/Jun and Ets protein families, respectively. Wild-type PyV restriction in PCC4 EC cells may be relieved by a cooperation between site 2- and 3-binding proteins that would thereby be activated. Since site 1- or 3-binding factors could be derepressed, we improved the analysis of UV cross-linked DNA-protein complexes and were able to detect a novel factor, called PEA1/2 (for PyV enhancer A site 1- and 2-binding factor). Its DNA binding sequence overlaps sites 1 and 2 (PEA2 binding site) and is not duplicated in the M1 mutant, which exhibits the highest Py EC-PCC4 enhancer activity. he suggest that PEA1/2 is also involved in the regulation of PyV enhancer activity by repressing the site 1-binding activity.

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