A NOVEL TRANSCRIPTION INITIATION-FACTOR (TIF), TIF-IE, IS REQUIRED FOR HOMOGENEOUS ACANTHAMOEBA-CASTELLANII TIF-IB (SL1) TO FORM A COMMITTED COMPLEX

The fundamental transcription initiation factor (TIF) for ribosomal RN A expression by eukaryotic RNA polymerase I, TIF-IB, has been purified to near homogeneity from Acanthamoeba castellanii using standard tech niques. The purified factor consists of the TATA-binding protein and f our TATA-binding protein-associated factors with relative molecular we ights of 145,000, 99,000, 96,000, and 91,000. This yields a calculated native molecular weight of 460,000, which compares well with its mass determined by scanning transmission electron microscopy (493,000) and its sedimentation rate, which is close to RNA polymerase I (515,000). Both impure and nearly homogeneous TIF-IB exhibit an apparent equilib rium dissociation constant of 56 +/- 3 pM. However, although impure TI F-IB can form a promoter-DNA complex resistant to challenge by other p romoter-containing DNAs, near homogeneous TIF-IB cannot do so. An addi tional transcription factor, dubbed TIF-IE, restores the ability of ne ar homogeneous TIF-IB to sequester DNA into a committed complex.