RNA editing and the mitochondrial cryptogenes of kinetoplastid protozoa.

The term “RNA editing” was initially introduced to describe a process resulting in the addition of nongenomitally encoded uridine residues (U’s) to mitochondrial mRNAs in the kinetoplastid protozoa (Benne et al., 1986), but has recently been used to describe other processes involving modification of RNA in other organisms (Powell et al., 1987; Chen et al., 1987; Thomas et al., 1988). We would like to define RNA editing in a broad sense as any process that results in the production of an RNA molecule which differs in nucleotide sequence in coding regions from the DNA template, provided that the difference is not the result of the removal of introns by a classic splicing mechanism. In this review we will examine RNA editing mainly in kinetoplastids but also in other systems and will critically evaluate the central issues and concepts related to this phenomenon. The discovery of RNA editing was the direct result of a series of investigations on the genomic organization and transcription of the unusual mitochondrial genome of the kinetoplastid protozoa. Since the initial report, several other examples of kinetoplastid RNA editing have been discovered (Feagin et al., 1987; Feagin et al., 1988a; Shaw et al., 1988; van der Spek et al., 1988; Feagin et al., 1988b; Abraham et al., 1988). To provide a biological perspective on the discovery and role of RNA editing in kinetoplastids, we will first review selected aspects of the molecular biology of kinetoplastid protozoa.