The sugar beet mitochondrial nad4 gene: an intron loss and its phylogenetic implication in the Caryophyllales

The sugar beet mitochondrial gene for subunit IV of NADH dehydrogenase ( nad4) has been characterized. Unlike the corresponding genes in wheat and turnip, sugar beet nad4 lacks the second intron ( nad4-i2). Northern-blot analysis demonstrates transcription of the gene. A total of 19 RNA editing sites were identified in the sugar beet nad4 transcripts; interestingly, there is no editing in the region which flanks the lost intron. This observation is in favour of intron loss via homologous recombination of an edited RNA intermediate. We also found that the nad4-i2 intron is absent from the mitochondrial genomes of all examined members of the Caryophyllales, but present in the closely related orders, Polygonales and Plumbaginales, which suggests that the intron was lost in the common ancestor of the Caryophyllales.