Transcript Editing in Chloroplasts of Higher Plants

Abstract

Posttranscriptional alterations of single nucleotides within an mRNA were first described for transcripts encoded by the kinetoplast DNA of trypanosomes (Benne et al. 1986). In this case, insertions and deletions of U residues directed by small guide RNAs can result in a substantial deviation of the mature mRNA sequence from the corresponding DNA template. This phenomenon was termed “RNA editing” by analogy to an editor’s job modifying a manuscript and delivering a ready-for-press version. Soon after their initial discovery in kinetoplastid mitochondria, editing processes were detected in a variety of other genetic systems, including the nuclei of mammalian cells (Chen et al. 1987; Powell et al. 1987; Sommer et al. 1991) and the mitochondria, and chloroplasts of higher plants (Covello and Gray 1989; Gualberto et al. 1989; Hiesel et al. 1989; Hoch et al. 1991). A summary of the RNA editing systems known to date is given in Table 1. It appears useful to formally distinguish between two major types of RNA editing: insertional/deletional and substitutional editing. The above-mentioned kinetoplastid editing is of the insertional/deletional type, whereas editing in mammalian nuclei, plant mitochondria, and chloroplasts is of the substitutional type. Editing in mitochondria of the slime mold Physarum polycephalum is exceptional since in this system a mixture of base conversions and nucleotide insertions occurs (Gott et al. 1993). The case of paramyxoviral editing belongs to the gray zone around RNA editing in the strict sense since the guanosine nucleotides are inserted co-transcriptionally owing to stuttering of the RNA polymerase (Thomas et al. 1988).