The higher plant nad5 mitochondrial gene: a conserved discontinuous transcription pattern.

The single copy nad5 gene has been identified in the mitochondrial genomes of cauliflower, chicory, potato, fennel, and common bean. In these five dicot species the same organization as in Oenothera, Arabidopsis, wheat, and maize has been found for the gene: it consists of five exons organized into three independent groups. The first group comprises exons I and II, separated by a highly conserved group II intron, while the second group consists of exon III only. In the third group exons IV and V are separated by a group II intron of variable, species-specific, length. Transcription analysis of the nad5 gene in chicory and in cauliflower shows that the five exons are assembled as a mature mRNA through intermolecular interactions and multiple splicing events. Comparison of transcription in the gene with that of wheat and maize suggests that a common mechanism exists in higher plants for nad5 transcript processing.