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Molecular and Cellular Biology, February 2002, p. 750-761, Vol. 22, No. 3
0270-7306/01/$04.00+0     DOI: 10.1128/MCB.22.3.750-761.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

A Novel RNA Polymerase I Transcription Initiation Factor, TIF-IE, Commits rRNA Genes by Interaction with TIF-IB, Not by DNA Binding

Anna Maria Al-Khouri,{dagger} and Marvin R. Paule*

Department of Biochemistry and Molecular Biology, Colorado State University, Fort Collins, Colorado 80523-1870

Received 18 July 2001/ Returned for modification 28 August 2001/ Accepted 1 November 2001

In the small, free-living amoeba Acanthamoeba castellanii, rRNA transcription requires, in addition to RNA polymerase I, a single DNA-binding factor, transcription initiation factor IB (TIF-IB). TIF-IB is a multimeric protein that contains TATA-binding protein (TBP) and four TBP-associated factors that are specific for polymerase I transcription. TIF-IB is required for accurate and promoter-specific initiation of rRNA transcription, recruiting and positioning the polymerase on the start site by protein-protein interaction. In A. castellanii, partially purified TIF-IB can form a persistent complex with the ribosomal DNA (rDNA) promoter while homogeneous TIF-IB cannot. An additional factor, TIF-IE, is required along with homogeneous TIF-IB for the formation of a stable complex on the rDNA core promoter. We show that TIF-IE by itself, however, does not bind to the rDNA promoter and thus differs in its mechanism from the upstream binding factor and upstream activating factor, which carry out similar complex-stabilizing functions in vertebrates and yeast, respectively. In addition to its presence in impure TIF-IB, TIF-IE is found in highly purified fractions of polymerase I, with which it associates. Renaturation of polypeptides excised from sodium dodecyl sulfate-polyacrylamide gels showed that a 141-kDa polypeptide possesses all the known activities of TIF-IE.

* Corresponding author. Mailing address: Department of Biochemistry and Molecular Biology, Colorado State University, Fort Collins, CO 80523-1870. Phone: (970) 491-6748. Fax: (970) 491-0494. E-mail: mpaule{at}lamar.colostate.edu.

{dagger} Present address: Ludwig Institute for Cancer Research, San Diego, California.

Molecular and Cellular Biology, February 2002, p. 750-761, Vol. 22, No. 3
0022-538X/01/$04.00+0     DOI: 10.1128/MCB.22.3.750-761.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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