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Molecular and Cellular Biology, December 2006, p. 8781-8790, Vol. 26, No. 23
0270-7306/06/$08.00+0     doi:10.1128/MCB.01409-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Chromatin Structure and Transcription of the R1- and R2-Inserted rRNA Genes of Drosophila melanogaster{triangledown}

Junqiang Ye{dagger} and Thomas H. Eickbush*

University of Rochester, Department of Biology, Rochester, New York 14627

Received 1 August 2006/ Accepted 1 September 2006

About half of the rRNA gene units (rDNA units) of Drosophila melanogaster are inserted by the retrotransposable elements R1 and R2. Because transcripts to R1 and R2 were difficult to detect on blots and electron microscopic observations of rRNA synthesis suggested that only uninserted rDNA units were transcribed, it has long been postulated that inserted rDNA units are in a repressed (inactive) chromatin structure. Studies described here suggest that inserted and uninserted units are equally accessible to DNase I and micrococcal nuclease and contain similar levels of histone H3 and H4 acetylation and H3K9 methylation. These studies have low sensitivity, because psoralen cross-linking suggested few (estimated <10%) of the rDNA units of any type are transcriptionally active. Nuclear run-on experiments revealed that R1-inserted and R2-inserted units are activated for transcription at about 1/5 and 1/10, respectively, the rate of uninserted units. Most transcription complexes of the inserted units terminate within the elements, thus explaining why previous molecular and electron microscopic methods indicated inserted units are seldom transcribed. The accumulating data suggest that all units within small regions of the rDNA loci are activated for transcription, with most control over R1 and R2 activity involving steps downstream of transcription initiation.

* Corresponding author. Mailing address: Department of Biology, University of Rochester, Rochester, NY 14627. Phone: (585) 275-7247. Fax: (585) 275-2070. E-mail: eick{at}mail.rochester.edu.

{triangledown} Published ahead of print on 25 September 2006.

{dagger} Present address: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138.

Molecular and Cellular Biology, December 2006, p. 8781-8790, Vol. 26, No. 23
0270-7306/06/$08.00+0     doi:10.1128/MCB.01409-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Zhang, X., Eickbush, M. T., Eickbush, T. H. (2008). Role of Recombination in the Long-Term Retention of Transposable Elements in rRNA Gene Loci. Genetics 180: 1617-1626 [Abstract] [Full Text]  
  • Eickbush, D. G., Ye, J., Zhang, X., Burke, W. D., Eickbush, T. H. (2008). Epigenetic Regulation of Retrotransposons within the Nucleolus of Drosophila. Mol. Cell. Biol. 28: 6452-6461 [Abstract] [Full Text]  


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