Nucleolin Is Required for RNA Polymerase I Transcription In Vivo

Abstract

Eukaryotic genomes are packaged with histones and accessory proteins in the form of chromatin. RNA polymerases and their accessory proteins are sufficient for transcription of naked DNA, but not of chromatin, templates in vitro. In this study, we purified and identified nucleolin as a protein that allows RNA polymerase II to transcribe nucleosomal templates in vitro. As immunofluorescence confirmed that nucleolin localizes primarily to nucleoli with RNA polymerase I, we demonstrated that nucleolin allows RNA polymerase I transcription of chromatin templates in vitro. The results of chromatin immunoprecipitation experiments established that nucleolin is associated with chromatin containing rRNA genes transcribed by RNA polymerase I but not with genes transcribed by RNA polymerase II or III. Knockdown of nucleolin by RNA interference resulted in specific inhibition of RNA polymerase I transcription. We therefore propose that an important function of nucleolin is to permit RNA polymerase I to transcribe nucleolar chromatin.

We thank Paul Schedl and Thomas Shenk for invaluable discussion. We also thank Les Hanakahi, Ingrid Grummt, and Renate Voit for the generous gifts of plasmids.

This work was supported by grants from the NIH, and B.R. was supported by a predoctoral fellowship from the N.J. Commission on Cancer Research.