ACANTHAMOEBA-CASTELLANII CONTAINS A RIBOSOMAL-RNA ENHANCER-BINDING PROTEIN WHICH STIMULATES TIF-IB BINDING AND TRANSCRIPTION UNDER STRINGENT CONDITIONS

The intergenic spacer (IGS) of Acanthamoeba castellanii rRNA genes con tains repeated elements which are weak enhancers for transcription by RNA polymerase I, A protein, EBF, was identified and partially purifie d which binds to the enhancers and to several other sequences within t he IGS, but not to other DNA fragments, including the rRNA core promot er, No consensus binding sequence could be discerned in these fragment s and bound factor is in rapid equilibrium with unbound, EBF has funct ional characteristics similar to vertebrate upstream binding factors ( UBF), Not only does it bind to the enhancer and other IGS elements, bu t it also stimulates binding of TIF-IB, the fundamental transcription initiation factor, to the core promoter and stimulates transcription f rom the promoter, Attempts to identify polypeptides with epitopes simi lar to rat or Xenopus laevis UBF suggest that structurally the protein from A. castellanii is not closely related to vertebrate UBF.