EDITING DOMAINS OF TRYPANOSOMA-BRUCEI MITOCHONDRIAL RNAS IDENTIFIED BY SECONDARY STRUCTURE

The posttranscriptional insertion and deletion of U residues in trypan osome mitochondrial transcripts called RNA editing initiates at the 3' end of precisely defined editing domains that can be identified indep endently of the cognate guide RNA, The regions where editing initiates in Trypanosoma brucei cytochrome b and cytochrome oxidase subunit II preedited mRNAs are specifically cleaved by a trypanosome mitochondria l endonuclease that acts like mung bean nuclease and therefore is sing le strand specific, The regions where editing initiates in virtually a ll examined preedited mRNAs are predicted to form loop structures, sug gesting that editing domains could generally be recognized as prominen t single-stranded loops. In contrast to preedited mRNA, edited mRNA ca n be either resistant or sensitive to cleavage by trypanosome mitochon drial endonuclease, depending on the reaction conditions, This selecti vity appears dependent on the availability of extract RNAs, and in mod el reactions, edited mRNA becomes resistant to cleavage upon base pair ing with its guide RNA. Natural partially edited mRNAs are also specif ically cleaved with a sensitivity like preedited and unlike edited mRN As, consistent with their being intermediates in editing, These result s suggest that in vivo, the structure of editing domains could initial ly be recognized by the mitochondrial endonuclease, which could target its associated RNA ligase and terminal U transferase to begin cycles of enzymatic editing modifications.