MAXICIRCLE DNA AND EDITED MESSENGER-RNA SEQUENCES OF CLOSELY-RELATED TRYPANOSOME SPECIES - IMPLICATIONS OF KRNA EDITING FOR EVOLUTION OF MAXICIRCLE GENOMES

kRNA editing produces functional mRNAs by uridine insertion and deletion. We analyzed portions of the apocytochrome b and NADH dehydrogenase subunits 7 and 8 (ND7 and 8) genes and their edited mRNAs in Trypanosoma congolense and compared these to the corresponding sequences in T.brucei. We find that these genes are highly diverged between the two species, especially in the positions of thymidines and in nucleotide transitions. Editing eliminates differences in encoded uridines producing edited mRNAs that are identical except for the nucleotide substitutions. The resulting predicted proteins are identical since all nucleotide substitutions are silent. A T.congolense minicircle-encoded gRNA which can specify editing of ND8 mRNA was identified. This gRNA can basepair with both T.congolense and T.brucei ND8 mRNA despite nucleotide transitions due to the flexibility of G:U basepairing. These results illustrate how editing affects the characteristics of maxicircle sequence divergence and allows protein sequence conservation despite a level of DNA sequence divergence which would be predicted to be intolerable in the absence of editing.