Comparative structure and genomic organization of the discontinuous mitochondrial ribosomal RNA genes of Chlamydomonas eugametos and Chlamydomonas reinhardtii.

We report that the mitochondrial ribosomal RNAs (rRNAs) of Chlamydomonas eugametos are discontinuously encoded in separate gene pieces that are scrambled in order and interspersed with protein coding genes. Individual transcripts of these mitochondrial rRNA gene pieces have the potential to form standard rRNA secondary structures through intermolecular base-pairing and they all have termini that are confined to previously defined variable rRNA domains. The C. eugametos and the previously described Chlamydomonas reinhardtii mitochondrial DNAs, therefore, share the unusual feature of highly fragmented and extensively rearranged rRNA coding regions, which contrasts with the conventional mitochondrial rRNA gene structure of land plants and other green algae. Although many of the sites of mitochondrial rRNA discontinuity are in corresponding variable regions in the two Chlamydomonas species, several variable rRNA regions are interrupted in one species but not the other and the 5' to 3' order of the C. eugametos and C. reinhardtii gene pieces is very different. Based on these results, we conclude that the last common ancestor of C. eugametos and C. reinhardtii had discontinuous mitochondrial rRNA genes and that processes responsible for the further division and scrambling of these coding regions have continued since the divergence of C. eugametos and C. reinhardtii. The presence of four group I introns within the C. eugametos mitochondrial rRNA gene pieces leads us to favour recombination rather than reverse-transcription as the mechanism giving rise to the scrambled arrangement of rRNA genes in Chlamydomonas mitochondria.