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Molecular and Cellular Biology, July 2004, p. 6476-6487, Vol. 24, No. 14
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.14.6476-6487.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Diazaborine Treatment of Yeast Cells Inhibits Maturation of the 60S Ribosomal Subunit

Brigitte Pertschy, Gertrude Zisser, Hermine Schein, René Köffel, Gernot Rauch, Karlheinz Grillitsch, Christina Morgenstern, Michael Durchschlag, Gregor Högenauer,^* and Helmut Bergler

Institut für Molekularbiologie, Biochemie und Mikrobiologie, Karl-Franzens-Universität Graz, A-8010 Graz, Austria

Received 2 February 2004/ Returned for modification 8 March 2004/ Accepted 19 April 2004

Diazaborine treatment of yeast cells was shown previously to cause accumulation of aberrant, 3'-elongated mRNAs. Here we demonstrate that the drug inhibits maturation of rRNAs for the large ribosomal subunit. Pulse-chase analyses showed that the processing of the 27S pre-rRNA to consecutive species was blocked in the drug-treated wild-type strain. The steady-state level of the 7S pre-rRNA was clearly reduced after short-term treatment with the inhibitor. At the same time an increase of the 35S pre-rRNA was observed. Longer incubation with the inhibitor resulted in a decrease of the 27S precursor. Primer extension assays showed that an early step in 27S pre-rRNA processing is inhibited, which results in an accumulation of the 27SA2 pre-rRNA and a strong decrease of the 27SA3, 27SB1L, and 27SB1S precursors. The rRNA processing pattern observed after diazaborine treatment resembles that reported after depletion of the RNA binding protein Nop4p/Nop77p. This protein is essential for correct pre-27S rRNA processing. Using a green fluorescent protein-Nop4 fusion, we found that diazaborine treatment causes, within minutes, a rapid redistribution of the protein from the nucleolus to the periphery of the nucleus, which provides a possible explanation for the effect of diazaborine on rRNA processing.

Present address: Department of Medicine, Division of Biochemistry, University of Fribourg, CH-1700 Fribourg, Switzerland.

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