1. ¹Laboratoire d'Oncologie Moléculaire. EA3805, Pô le Biologie-Santé. 40, Poitiers cedex, France
2. ²Groupe de Recherche sur le Cancer du Poumon, EA 2021, Unité INSERM U578, Institut Albert Bonniot, La Tronche, France

Correspondence: Dr P Séité, Laboratoire d'Oncologie Moléculaire. EA3805, Pôle Biologie-Santé. 40, Avenue du Recteur Pineau, 86022 Poitiers cedex, France. E-mail: paule.seite@univ-poitiers.fr

Received 14 December 2005; Revised 19 April 2006; Accepted 10 May 2006; Published online 21 August 2006.

Abstract

The nucleolar Arf protein has been shown to regulate cell cycle through both p53-dependent and -independent pathways. In addition to the well-characterized Arf-mdm2-p53 pathway, several partners of Arf have recently been described that could participate in alternative regulation process. Among those is the nucleolar protein B23/NPM, involved in the sequential maturation of rRNA. p19^ARF can interact with B23/NPM in high molecular complexes and partially inhibit the cleavage of the 32S rRNA, whereas the human p14^ARF protein has been shown to participate in the degradation of NPM/B23 by the proteasome. These data led to define Arf as a negative regulator of ribosomal RNA maturation. Our recent finding that the human p14^ARF protein was able to specifically interact with the rRNA promoter in a p53-independent context, led us to analyse in vitro and in vivo the consequences of this interaction. Luciferase assay and pulse-chase experiments demonstrated that the rRNA transcription was strongly reduced upon p14^ARF overexpression. Investigations on potential interactions between p14^ARF and the transcription machinery proteins demonstrated that the upstream binding factor (UBF), required for the initiation of the transcriptional complex, was a new partner of the p14^ARF protein. We next examined the phosphorylation status of UBF as UBF phosphorylation is required to recruit on the promoter factors involved in the transcriptional complex. Upon p14^ARF overexpression, UBF was found hypophosphorylated, thus unable to efficiently recruit the transcription complex. Taken together, these data define a new p53-independent pathway that could regulate cell cycle through the negative control of rRNA transcription.