THE SPECIES-SPECIFIC RNA-POLYMERASE-I TRANSCRIPTION FACTOR SL-1 BINDSTO UPSTREAM BINDING-FACTOR

Transcription of the 45S rRNA genes is carried out by RNA polymerase I and at least two trans-acting factors, upstream binding factor (UBF) and SL-1. We have examined the hypothesis that SL-I and UBF interact. Coimmunoprecipitation studies using an antibody to UBF demonstrated th at TATA-binding protein, a subunit of SL-1, associates with UBF in the absence of DNA. Inclusion of the detergents sodium dodecyl sulfate an d deoxycholate disrupted this interaction. In addition, partially puri fied UBF from rat cell nuclear extracts and partially purified SL-1 fr om human cells coimmunoprecipitated with the anti-UBF antibody after m ixing, indicating that the UBF-SL-1 complex can re-form. Treatment of UBF-depleted extracts with the anti-UBF antibody depleted the extracts of SL-1 activity only if UBF was added to the extract prior to the im munodepletion reaction. Furthermore, SL-1 activity could be recovered in the immunoprecipitate. Interestingly, these immunoprecipitates did not contain RNA polymerase I, as a monospecific antibody to the 194-kD a subunit of RNA polymerase I failed to detect that subunit in the imm unoprecipitates. Treatment of N1S1 cell extracts with the anti-UBF ant ibody depleted the extracts of SL-1 activity but not TFIIIB activity, suggesting that the binding of UBF to SL-1 is specific and not solely mediated by an interaction between UBF and TATA-binding protein, which is also a component of TFIIIB. These data provide evidence that UBF a nd SL-1 interact.