Coordination of kRNA editing and polyadenylation in Trypanosoma brucei mitochondria: complete editing is not required for long poly(A) tract addition.

Mitochondrial RNAs in Trypanosoma brucei are post-transcriptionally modified by the addition and deletion of uridylate residues in a process called kRNA editing. Unedited, partially edited and fully edited RNAs exist in the steady-state RNA population. Previous experiments have demonstrated that T.brucei mitochondrial RNAs contain both short (approximately 20 nt) and long (120-200 nt) poly(A) tracts. However, it is unknown exactly what poly(A) tract lengths are present on unedited, partially edited and fully edited RNAs. To gain insight into the role of the poly(A) tract in T.brucei mitochondria, ribosomal protein S12 (RPS12) RNAs with short and long poly(A) tracts were purified by hybrid selection and analyzed by RT-PCR and DNA sequencing. Unedited RPS12 RNAs were found almost exclusively in populations with short poly(A) tracts. Both partially and fully edited RPS12 RNAs were found in populations with short and long poly(A) tracts. Therefore, there is a correlation between the presence of editing and the presence of the long poly(A) tract. Since a proportion of partially edited RPS12 RNAs contain long poly(A) tracts, it is unlikely that the long poly(A) tract is the sole signal for translation. Other implications for the role of polyadenylation in mitochondrial gene regulation are discussed.