Figure 1.

Arrangements of the genes nad1, nad2, and nad5, which encode subunits of the NADH dehydrogenase in Marchantia polymorpha (A) and angiosperm mitochondria (C). Coding regions of the nad gene reading frames are distributed over 5 exons (boxes) each in angiosperms (C). Conventional cis-arranged group II introns (lines) are accompanied by disrupted group II introns (broken lines) requiring one (nad2) or two (nad1, nad5) trans-splicings. Additional intron disruptions are observed for intron nad1 d/e in wheat upstream and in petunia downstream of the maturase reading frame, respectively (arrows). The single nad2 intron in Marchantia polymorpha has its homologue in angiosperms as intervening sequence between exons c and d (asterisk). Conserved exon primer sequences (arrowheads) were designed for regions conserved between the angiosperm and Marchantia polymorpha nucleotide sequences. Primer design tested in PCR amplification yields the expected products for noninterrupted reading frame fragments from Marchantia polymorpha genomic DNA and Oenothera berteriana cDNA (D). Experiments are designed to search for cis-splicing introns as potential progenitors of the trans-splicing introns as shown for a hypothetical nad2 gene structure in B.