ASSEMBLY OF TRANSCRIPTIONALLY ACTIVE RNA-POLYMERASE-I INITIATION-FACTOR SL1 FROM RECOMBINANT SUBUNITS

Initiation of ribosomal RNA synthesis by RNA polymerase I requires the promoter selec tivity factor SL1, which consists of the TATA-binding protein, TBP, and three associated factors, TAF(I)s 110, 63, and 48. H ere the in vivo and in vitro assembly of functional SL1 complexes from recombinant TAF(I)s and TBP are reported. Complexes containing TBP an d all three TAF(I)s were as active in supporting transcription from th e human ribosomal RNA gene promoter as endogenous SL1, whereas partial complexes without TBP did not efficiently direct transcription in vit ro. These results suggest that TAF(I)s 110, 63, and 48, together with TBP, are necessary and sufficient to reconstitute a transcriptionally active SL1 complex.