Transcription of protein-coding genes in trypanosomes by RNA polymerase I

Annu Rev Microbiol. 1997;51:463-89. doi: 10.1146/annurev.micro.51.1.463.


In eukaryotes, RNA polymerase (pol) II transcribes the protein-coding genes, whereas RNA pol I transcribes the genes that encode the three RNA species of the ribosome [the ribosomal RNAs (rRNAs)] at the nucleolus. Protozoan parasites of the order Kinetoplastida may represent an exception, because pol I can mediate the expression of exogenously introduced protein-coding genes in these single-cell organisms. A unique molecular mechanism, which leads to pre-mRNA maturation by trans-splicing, facilitates pol I-mediated protein-coding gene expression in trypanosomes. Trans-splicing adds a capped 39-nucleotide mini-exon, or spliced leader transcript, to the 5' end of the main coding exon posttranscriptionally. In other eukaryotes, the addition of a 5' cap, which is essential for mRNA function, occurs exclusively as a result of RNA pol II-mediated transcription. Given the assumption that cap addition represents the limiting factor, trans-splicing may have uncoupled the requirement for RNA pol II-mediated mRNA production. A comparison of the alpha-amanitin sensitivity of transcription in naturally occurring trypanosome protein-coding genes reveals that a unique subset of protein-coding genes-the variant surface glycoprotein (VSG) expression sites and the procyclin or the procyclic acidic repetitive protein (PARP) genes-are transcribed by an RNA polymerase that is resistant to the mushroom toxin alpha-amanitin, a characteristic of transcription by RNA pol I. Promoter analysis and a pharmacological characterization of the RNA polymerase that transcribes these genes have strengthened the proposal that the VSG expression sites and the PARP genes represent naturally occurring protein-coding genes that are transcribed by RNA pol I.

Publication types

  • Review

MeSH terms

  • Amanitins / genetics
  • Animals
  • Gene Expression Regulation
  • Membrane Glycoproteins / genetics
  • Membrane Glycoproteins / metabolism
  • Promoter Regions, Genetic
  • Protozoan Proteins / genetics*
  • Protozoan Proteins / metabolism*
  • RNA Polymerase I / metabolism*
  • RNA Processing, Post-Transcriptional
  • RNA, Messenger / metabolism
  • RNA, Ribosomal / genetics
  • Transcription, Genetic*
  • Trypanosoma / genetics*
  • Trypanosoma / growth & development
  • Trypanosoma / metabolism*
  • Trypanosoma brucei brucei / genetics
  • Trypanosoma brucei brucei / metabolism


  • Amanitins
  • Membrane Glycoproteins
  • Protozoan Proteins
  • RNA, Messenger
  • RNA, Ribosomal
  • procyclic acidic repetitive protein, Trypanosoma
  • RNA Polymerase I