TRANSCRIPTION OF PROTEIN- CODING GENES IN TRYPANOSOMES BY RNA POLYMERASE I

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 α-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 α-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.