Developmentally regulated addition of nucleotides within apocytochrome b transcripts in Trypanosoma brucei.

Production of the mitochondrial respiratory system and expression of mitochondrial genes are developmentally regulated in T. brucei. We have found three classes of apocytochrome b (CYb) transcripts. Two of these contain U's within their 5' sequence that are not found in the CYb gene. They occur in procyclic forms, which have a complete mitochondrial respiratory system, but are absent in bloodstream forms, which lack this respiratory system. The third transcript has the sequence predicted from the genomic sequence and is present in both life cycle stages. These findings suggest that there is developmentally regulated posttranscriptional addition of U's to CYb transcripts. This process may contribute to the developmentally regulated production of the mitochondrial respiratory system.