Modifications of both selectivity factor and upstream binding factor contribute to poliovirus-mediated inhibition of RNA polymerase I transcription

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Abstract
Soon after infection, poliovirus (PV) shuts off host-cell transcription, which is catalysed by all three cellular RNA polymerases. rRNA constitutes more than 50 % of all cellular RNA and is transcribed from rDNA by RNA polymerase I (pol I). Here, evidence has been provided suggesting that both pol I transcription factors, SL-1 (selectivity factor) and UBF (upstream binding factor), are modified and inactivated in PV-infected cells. The viral protease 3Cproappeared to cleave the TATA-binding protein-associated factor 110 (TAF110), a subunit of the SL-1 complex, into four fragmentsin vitro.In vitroprotease-cleavage assays using various mutants of TAF110and purified 3Cproindicated that the Q265G266and Q805G806sites were cleaved by 3Cpro. Both SL-1 and UBF were depleted in PV-infected cells and their disappearance correlated with pol I transcription inhibition. rRNA synthesis from a template containing a human pol I promoter demonstrated that both SL-1 and UBF were necessary to restore pol I transcription fully in PV-infected cell extracts. These results suggested that both SL-1 and UBF are transcriptionally inactivated in PV-infected HeLa cells.