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Letter

Human ribosomal RNA gene arrays display a broad range of palindromic structures

¹ Unité de Stabilité des Génomes, Institut Pasteur, 75724 Paris, France ² Département de Biochimie, Université de Genève, CH-1211 Geneva 4, Switzerland

The standard model of eukaryotic ribosomal RNA (rRNA) genes involves tandem arrays with hundreds of units in clusters, the nucleolus organizer regions (NORs). A first genomic overview for human cells is reported here for these regions, which have never been sequenced in their totality, by using molecular combing. The rRNA-coding regions are examined by fluorescence on single molecules of DNA with two specific probes that cover their entire length. The standard organization assumed for rDNA units is a transcribed region followed by a nontranscribed spacer. While we confirmed this arrangement in many cases, unorthodox patterns were also observed in normal individuals, with one-third of the rDNA units rearranged to form apparently palindromic structures (noncanonical units) independent of the age of the donors. In cells from individuals with a deficiency in the WRN RecQ helicase (Werner syndrome), the proportion of palindromes increased to one-half. These findings, supported by Southern blot analyses, show that rRNA genes are a mosaic of canonical and (presumably nonfunctional) palindromic units that may be altered by factors associated with genomic instability and pathology.

Article and publication are at http://www.genome.org/cgi/doi/10.1101/gr.3970105. Article published online before print in July 2005.

³ These two authors contributed equally to this work.

⁴ Present address: INSERM U709, Hôpital Cochin, 75014 Paris, France.

⁵ Corresponding author.
E-mail abensim{at}pasteur.fr.; fax +33-1-45-68-87-90.

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