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A pol I transcriptional body associated with VSG mono-allelic expression in Trypanosoma brucei


In the mammalian host, African trypanosomes generate consecutive waves of parasitaemia by changing their antigenic coat. Because this coat consists of a single type of variant surface glycoprotein (VSG), the question arises of how a trypanosome accomplishes the transcription of only one of a multi-allelic family of VSG expression site loci to display a single VSG type on the surface at any one time1. No major differences have been detected between the single active expression site and the cohort of inactive expression sites2. Here we identify an extranucleolar body containing RNA polymerase I (pol I) that is transcriptionally active and present only in the bloodstream form of the parasite. Visualization of the active expression site locus by tagging with green fluorescent protein3 shows that it is specifically located at this unique pol I transcriptional factory. The presence of this transcriptional body in postmitotic nuclei and its stability in the nucleus after DNA digestion provide evidence for a coherent structure. We propose that the recruitment of a single expression site and the concomitant exclusion of inactive loci from a discrete transcriptional body define the mechanism responsible for VSG mono-allelic expression.

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Figure 1: Identification of an extranucleolar body containing pol I in bloodstream-form nuclei of T. brucei.
Figure 2: The pol I extranucleolar body has a different function from that of the nucleolus.
Figure 3: Br-UTP labelling of nascent RNA in permeabilized bloodstream-form nuclei indicates that the extranucleolar pol I structure is transcriptionally active.
Figure 4: GFP–LacI tagging of the active expression site (ES) sequences in bloodstream-form nuclei reveals the association of this locus with α-amanitin-resistant transcription and with the extranucleolar pol I body.


  1. Cross, G. A. M. Identification, purification and properties of variant-specific glycoprotein antigens constituting the surface coat of Trypanosoma brucei. Parasitology 71, 393–417 (1975).

    Article  CAS  Google Scholar 

  2. Borst, P. & Ulbert, S. Control of VSG gene expression sites. Mol. Biochem. Parasitol. 114, 17–27 (2001).

    Article  CAS  Google Scholar 

  3. Robinett, C. C. et al. In vivo localization of DNA sequences and visualization of large-scale chromatin organization using lac operator/repressor recognition. J. Cell Biol. 135, 1685–700 (1996).

    Article  CAS  Google Scholar 

  4. Pays, E., Lips, S., Nolan, D., Vanhamme, L. & Perez-Morga, D. The VSG expression sites of Trypanosoma brucei: multipurpose tools for the adaptation of the parasite to mammalian hosts. Mol. Biochem. Parasitol. 114, 1–16 (2001).

    Article  CAS  Google Scholar 

  5. Bitter, W., Gerrits, H., Kieft, R. & Borst, P. The role of transferrin-receptor variation in the host range of Trypanosoma brucei. Nature 391, 499–502 (1998).

    Article  ADS  CAS  Google Scholar 

  6. Pays, E. et al. The genes and transcripts of an antigen expression site from T. brucei. Cell 57, 835–845 (1989).

    Article  CAS  Google Scholar 

  7. Zomerdijk, J. C. B. M. et al. The promoter for a variant surface glycoprotein gene expression site in Trypanosoma brucei. EMBO J. 9, 2791–2801 (1990).

    Article  CAS  Google Scholar 

  8. Borst, P. & Chaves, I. Mono-allelic expression of genes in simple eukaryotes. Trends Genet. 15, 95–96 (1999).

    Article  CAS  Google Scholar 

  9. Robinson, N. P., Burman, N., Melville, S. E. & Barry, J. D. Predominance of duplicative VSG gene conversion in antigenic variation in African trypanosomes. Mol. Cell. Biol. 19, 5839–5846 (1999).

    Article  CAS  Google Scholar 

  10. Roditi, I. et al. Procyclin gene expression and loss of the variant surface glycoprotein during differentiation of Trypanosoma brucei. J. Cell Biol. 108, 737–746 (1989).

    Article  CAS  Google Scholar 

  11. Kooter, J. M. & Borst, P. Alpha-amanitin-insensitive transcription of variant surface glycoprotein genes provides further evidence for discontinuous transcription in trypanosomes. Nucleic Acids Res. 12, 9457–9472 (1984).

    Article  CAS  Google Scholar 

  12. Rudenko, G., Bishop, D., Gottesdiener, K. & Van der Ploeg, L. H. T. Alpha-amanitin resistant transcription of protein coding genes in insect and bloodstream form Trypanosoma brucei. EMBO J. 8, 4259–4263 (1989).

    Article  CAS  Google Scholar 

  13. Rudenko, G., Lee, M. G.-S. & Van der Ploeg, L. H. T. The PARP and VSG genes of Trypanosoma brucei do not resemble RNA polymerase II transcription units in sensitivity to Sarkosyl in nuclear run-on assays. Nucleic Acids Res. 20, 303–306 (1992).

    Article  CAS  Google Scholar 

  14. Laufer, G., Schaaf, G., Bollgonn, S. & Günzl, A. In vitro analysis of alpha-amanitin-resistant transcription from the rRNA, procyclic acidic repetitive protein, and variant surface glycoprotein gene promoters in Trypanosoma brucei. Mol. Cell. Biol. 19, 5466–5473 (1999).

    Article  CAS  Google Scholar 

  15. Scheer, U. & Hock, R. Structure and function of the nucleolus. Curr. Opin. Cell Biol. 11, 385–390 (1999).

    Article  CAS  Google Scholar 

  16. Hartshorne, T. & Agabian, N. RNA B is the major nucleolar trimethylguanosine-capped small nuclear RNA associated with fibrillarin and pre-rRNAs in Trypanosoma brucei. Mol. Cell. Biol. 13, 144–154 (1993).

    CAS  PubMed  PubMed Central  Google Scholar 

  17. Pombo, A. et al. Regional specialization in human nuclei: visualization of discrete sites of transcription by RNA polymerase III. EMBO J. 18, 2241–2253 (1999).

    Article  CAS  Google Scholar 

  18. Zomerdijk, J. C. B. M., Kieft, R. & Borst, P. Efficient production of functional mRNA mediated by RNA polymerase I in Trypanosoma brucei. Nature 353, 772–775 (1991).

    Article  ADS  CAS  Google Scholar 

  19. Chaves, I. et al. Subnuclear localization of the active variant surface glycoprotein gene expression site in Trypanosoma brucei. Proc. Natl Acad. Sci. USA 95, 12328–12333 (1998).

    Article  ADS  CAS  Google Scholar 

  20. Wirtz, E., Leal, S., Ochatt, C. & Cross, G. A. M. A tightly regulated inducible expression system for conditional gene knock-outs and dominant-negative genetics in Trypanosoma brucei. Mol. Biochem. Parasitol. 99, 89–101 (1999).

    Article  CAS  Google Scholar 

  21. Chaves, I., Rudenko, G., Dirks-Mulder, A., Cross, M. & Borst, P. Control of variant surface glycoprotein gene-expression sites in Trypanosoma brucei. EMBO J. 18, 4846–4855 (1999).

    Article  CAS  Google Scholar 

  22. Vanhamme, L. et al. Differential RNA elongation controls the variant surface glycoprotein gene expression sites of Trypanosoma brucei. Mol. Microbiol. 36, 328–340 (2000).

    Article  CAS  Google Scholar 

  23. Navarro, M., Cross, G. A. & Wirtz, E. Trypanosoma brucei variant surface glycoprotein regulation involves coupled activation/inactivation and chromatin remodeling of expression sites. EMBO J. 18, 2265–2272 (1999).

    Article  CAS  Google Scholar 

  24. Grosveld, F. Activation by locus control regions? Curr. Opin. Genet. Dev. 9, 152–157 (1999).

    Article  CAS  Google Scholar 

  25. Cook, P. R. The organization of replication and transcription. Science 284, 1790–1795 (1999).

    Article  CAS  Google Scholar 

  26. Ohlsson, R., Tycko, B. & Sapienza, C. Monoallelic expression: ‘there can only be one’. Trends Genet. 14, 435–438 (1998).

    Article  CAS  Google Scholar 

  27. Matthews, K. R. & Gull, K. Evidence for an interplay between cell cycle progression and the initiation of differentiation between life cycle forms of African trypanosomes. J. Cell Biol. 125, 1147–1156 (1994).

    Article  CAS  Google Scholar 

  28. Ersfeld, K. & Gull, K. Partitioning of large and minichromosomes in Trypanosoma brucei. Science 276, 611–614 (1997).

    Article  CAS  Google Scholar 

  29. Navarro, M. & Cross, G. A. DNA rearrangements associated with multiple consecutive directed antigenic switches in Trypanosoma brucei. Mol. Cell. Biol. 16, 3615–3625 (1996).

    Article  CAS  Google Scholar 

  30. Navarro, M. & Cross, G. A. In situ analysis of a variant surface glycoprotein expression-site promoter region in Trypanosoma brucei. Mol. Biochem. Parasitol. 94, 53–66 (1998).

    Article  CAS  Google Scholar 

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We thank A. F. Straight for the GFP–LacI tagging constructs; E. Wirtz and C. Ochatt for pLew100 and the SAT-derived construct; M. Hoek and G. A. M. Cross for the BAC clone; G. Pierron for the fibrillarin (P2G3) monoclonal antibody; and K. E. Sawin for the anti-GFP rabbit polyclonal antibody. We are grateful to K. Ersfeld for FISH protocols, and D. Robinson for technical advice. We thank A. Baines for technical assistance at the initial stage of this work, and all members of the Gull laboratory for discussions. This work was founded by the Wellcome Trust and the BBSRC.

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Correspondence to Miguel Navarro.

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Movie (MOV 1.2 MB)

Immunofluorescence analysis of the bloodstream form of T. brucei using an anti-pol-I antibody (red) and DAPI staining (blue). The three-dimensional nature of the pol I body in the nucleoplasm is shown.

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Navarro, M., Gull, K. A pol I transcriptional body associated with VSG mono-allelic expression in Trypanosoma brucei. Nature 414, 759–763 (2001).

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