Conserved genes encode guide RNAs in mitochondria of Crithidia fasciculata

RNA editing is the post-transcriptional alteration of the nucleotide sequence of RNA, which in trypanosome mitochondria is characterized by the insertion and deletion of uridine residues. It has recently been proposed that the information for the sequence alteration in Leishmania tarentolae is provided by small guide (g) RNAs encoded in the mitochondrial DNA [Blum et al. (1990) Cell, 60, 189-198]. We are studying the mechanism of RNA editing in the insect trypanosome Crithidia fasciculata and report that: (i) a full length, conventional DNA gene or an independently replicating RNA gene that could encode the edited MURF3 transcript is absent when probed for in sensitive, calibrated assay systems; (ii) in all cases (seven) investigated in C. fasciculata so far, putative gRNA genes are found in a position in the mitochondrial DNA virtually identical to that in L. tarentolae and (iii) also in C. fasciculata, the putative gRNA genes are transcribed into small RNAs with discrete 5' ends. These results provide strong evolutionary evidence in support of the participation of gRNAs in RNA editing. Remarkably, in C. fasciculata the basepaired region of some putative gRNA:mRNA hybrids contains a C:A non-Watson-Crick basepair.