The transcription factor EGR-1 localizes in the nucleolus and controls 47S precursor ribosomal RNA. D. Ponti, R. Puca, G.C. Bellenchi, P. Ruggieri, L. Pacini, M. Maroder, G. Ragona, P. Roussel, A. Calogero, Dept. of Medical-Surgical Sciences and Biotechnologies, Sapienza University, Latina, Italy, 2 Institute of Genetics and Biophysics, Naples, Italy, 3Université Paris Diderot, Paris, FranceBackground: The early grow factor EGR-1 is involved in various functions such as regulation of proliferation and differentiation in response to different stimuli and is down regulated in many cancer cell types. Re-introduction of EGR-1 in EGR-1 deficient cell lines, suppresses transformation and tumorigenicity suggest - ing a potential role as tumor suppressor. It has been proposed that the interaction with ARF is an important event for regulat - ing EGR-1 downstream targets expression. Observations: In the present study we investigated whether EGR-1 localizes in the nucleolus, similarly to what reported for ARF. We observed, by confocal microscopy, that the endogenous EGR-1 colocalizes, in the nucleolar compartment with specific markers such as fibrillarin and B23. By generating truncated forms of the protein, fused to the GFP, we found that the deletion of a DNA binding domain at the C-terminus of the protein impairs EGR-1 localization, suggesting a role as nucleolar localization signal for this region. The nucleolar localization was confirmed by western blot in nucleolar extracts obtained from HeLa cells. Because the main function of the nucleolus is the ribosome bio - genesis we investigated, by real-time PCR, whether modulation of EGR-1 affects nucleolar metabolism. By overexpression and silencing we observed an inverse correlation between EGR-1 and the pre-rRNA 47S, thus suggesting a potential control on ribosomal RNA precursor. Conclusions: In this work we present, for the first time, data showing that EGR-1 localizes in the nucleolus and affects nucleolar metabolism, thus suggesting a potential novel mechanism to control cell growth