Fig. 8. p42^POP modulates transcriptional activity. HeLa cells were transfected with the reporter plasmid pGL3-mreA-Luc and a cDNA encoding c-Myb, BiPro-p42^POP or BiPro-POP4. (A) Immunoblot of cell extracts with the anti-BiPro antibody after transfection reveals comparable expression of p42^POP and the POP4 fragment. (B) Luciferase activity in extracts of cells transfected with the reporter plasmid only (control) or with the reporter plus c-Myb, BiPro-p42^POP or BiPro-POP4. Whereas c-Myb acts as a transcriptional activator in this assay, p42^POP causes a decrease in transcription to less than 15% of the control value. By contrast, POP4, lacking the C-terminal modules, induces a threefold increase in transcription.