IN-VIVO EVIDENCE THAT TATA-BINDING PROTEIN SL1 COLOCALIZES WITH UBF AND RNA-POLYMERASE-I WHEN RIBOSOMAL-RNA SYNTHESIS IS EITHER ACTIVE OR INACTIVE

Here we show that the TATA-binding protein (TBP) is localized in the n ucleoplasm and in the nucleolus of mammalian cells, consistent with it s known involvement in transcription by RNA polymerase I, II, and III. In the nucleolus of actively glowing cells, TBP colocalizes with upst ream binding factor (UBF) and RNA polymerase I at the sites of rRNA tr anscription, During mitosis, when rRNA synthesis is down-regulated, TB P colocalizes with TBP-associated factors for RNA polymerase I (TAF(I) s), UBF, and RNA polymerase I on the chromosomal regions containing th e rRNA genes. Treatment of cells with a low concentration of actinomyc in D inhibits rRNA synthesis and causes a redistribution of the rRNA g enes that become concentrated in clusters at the periphery of the nucl eolus, A similar redistribution was observed for the major components of the rRNA transcription machinery (i.e., TBP, TAF(I)s, UBF, and RNA polymerase I), which still colocalized with each other. Furthermore, a nti-TBP antibodies are shown to coimmunoprecipitate TBP and TAF(I)63 i n extracts prepared from untreated and actinomycin D-treated cells, Co llectively, the data indicate that in vivo TBP/promoter selectivity fa ctor, UBF, and RNA polymerase I remain associated with both active and inactive rRNA genes.