Trypanosoma equiperdum Minicircles Encode Three Distinct Primary Transcripts Which Exhibit Guide RNA Characteristics

Abstract

The mitochondrial DNA of trypanosomes is composed of maxicircle and minicircle DNAs catenated into a network, called the kinetoplast. Maxicircles encode proteins and RNAs necessary for mitochondrial assembly. Minicircles encode small transcripts which are believed to serve as guide RNAs in the process of RNA editing of maxicircle transcripts. Trypanosoma equiperdum minicircles contain three transcription units which produce three distinct transcripts. The genes for these transcripts are flanked by imperfect 18-bp repeats separated by approximately 110 bp. The transcripts have a 5′ triphosphate, indicating that they are primary transcripts. Minicircle transcription initiates at a purine within a conserved sequence, 5′-AYAYA-3′, where Y is a pyrimidine, 32 bp from the upstream inverted repeat, suggesting that the repeats may function in transcript initiation. Transcripts from a single minicircle transcription unit range in size from 55 to 70 nucleotides. This size heterogeneity within a single sequence class is due to the variable length of nontemplated uridine residues composing a 3′ tail. The size range and heterogeneous polyuridylate 3′ end of the minicircle transcripts appear to be conserved features and may be related to transcript function.