| dc.description | The mitochondrial rRNAs of the green alga Chlamydomonas eugametos are discontinuously encoded in separate gene pieces that are scrambled in order and interspersed with protein coding genes. Southern and northern hybridization analyses indicate that the mitochondrial rRNA genes of C. eugametos are similar in structure and organization to those of its interfertile close relative Chlamydomonas moewusii but different in many respects to the previously described fragmented and scrambled mitochondrial rRNA gene pieces of Chlamydomonas reinhardtii. The mitochondrial rRNA genes of these three Chlamydomonas algae, therefore, contrast with the conventional mitochondrial rRNA genes found in land plants and other green algae. Three lines of evidence support the hypothesis that the discontinuous and scrambled gene pieces encode the functional rRNAs in Chlamydomonas mitochondria. First, no continuous rRNA genes exist in the mitochondrial DNA of the three Chlamydomonas algae. Second, the individual transcripts of these mitochondrial rRNA gene pieces have termini that are confined to previously defined variable rRNA domains and the rRNA transcripts have the potential to form standard rRNA secondary structures through inter-molecular base-pairing. Third, mitochondrial ribosomes and ribosomal subunits were identified by northern blot hybridization of fractionated C. eugametos cellular ribosomes. Based on a detailed comparative analysis of the mitochondrial rRNA genes in C. eugametos and C. reinhardtii, calculations of the minimal number of transpositions required to convert hypothetical ancestral rRNA gene organizations to the arrangements observed for C. eugametos and C. reinhardtii and on a limited survey of the size of mitochondrial rRNAs in other members of the genus, it appears that the last common ancestor of Chlamydomonas algae had fragmented mitochondrial rRNA genes that were co-linear with conventional rRNA genes and that separate processes responsible for the further division of the rRNA genes into separate coding regions and the scrambling of these coding regions have occurred since the divergence of the major lineages leading to C. eugametos and C. reinhardtii. However, because phylogenetic analysis did not reveal a specific affiliation between the mitochondrial rRNA gene sequences of Chlamydomonas and other green algae, land plants or bacteria, it is not possible at this time to rule out the possibility that the mitochondrial rRNA genes of Chlamydomonas had an independent evolutionary origin from those of other green algae and land plants. | en_US |
