Figure 1.

Phylogenetic position of diplonemids and their mitochondrial DNA structure and gene expression. (A) Recent classification of eukaryotes (based on (1)) highlighting the position of diplonemids. (B) The various steps of mitochondrial gene expression in diplonemids. The model gene consists of three pieces, also referred to as modules, each encoded in a unique region (cassette) on a different chromosome. Modules, together with surrounding regions, are transcribed separately from a promoter located in the constant region (25). Primary transcripts are end-processed, removing 5′ and 3′ non-coding regions from the primary transcripts. Certain module transcripts undergo substitution RNA editing and/or appendage RNA editing (nucleotide additions at the module's 3′ end). The module transcript that will constitute the transcript’s 3′ end is poly-adenylated (mRNAs and mtLSU rRNA) or poly-uridylated (mtSSU rRNA) (16,19). Finally, modules are joined together (trans-spliced) yielding mature RNA (mRNA or rRNA). Note that all post-transcriptional processes (gray background) occur in parallel in the diplonemid mitochondrion (19); thus, the arrows do not imply strict sequentiality.