Joachim R. Marienfeld

The molecular basis of the maternally inherited, heteroplasmic NCS2 mutant of maize was investigated. Analysis of the NCS2 mtDNA showed that it closely resembles the progenitor cmsT mitochondrial genome, except that the mutant genome contains a fused nad4-nad7 gene and is deleted for the small fourth exon of nad4. The rearrangement has occurred at a 16-bp repeat present in the third intron of the nad4 gene and in the second intron of the nad7 gene. Transcripts containing exon 4 of the nad4 gene are greatly reduced in mtRNA preparations from heteroplasmic NCS2 plants; larger transcripts are associated with the first three nad4 exons. Identical 5' ends of the nad4 transcripts have been mapped 396 and 247 bp upstream of the start codon in mtRNAs from both NCS2 and related non-NCS plants. The putative transcription termination signal of nad4 is deleted in mutant DNA, resulting in the production of the unique longer transcripts. The complex transcript pattern associated with nad7 is also altered in the mutant. Both nad4 and nad7 encode subunits of complex I (NADH dehydrogenase) of the mitochondrial electron transfer chain. Oxygen uptake experiments show that the functioning of complex I is specifically reduced in mitochondria isolated from NCS2 mutant plants.

Several retrotransposon fragments are integrated in the mitochondrial genome of Arabidopsis thaliana. These insertions are derived from all three classes of nuclear retrotransposons, the Ty1/copia-, Ty3/gypsy- and non-LTR/LINE-families. Members of the Ty3/gypsy group of elements have not yet been identified in the nuclear genome of Arabidopsis. The varying degrees of similarity with nuclear elements and the dispersed locations of the sequences in the mitochondrial genome suggest numerous independent transfer-insertion events in the evolutionary history of this plant mitochondrial genome. Overall, we estimate remnants of retrotransposons to cover > or = 5% of the mitochondrial genome in Arabidopsis.