The Mitochondrial LSU rDNA of the Brown AlgaPylaiella littoralisReveals α-Proteobacterial Features and is Split by Four Group IIB Introns with an Atypical Phylogeny

The mitochondrial DNA region coding for the larger ribosomal RNA subunit from the brown algaPylaiella littoralis(L.) Kjellm was sequenced. The LSU rRNA was folded into a secondary structure and aligned with homologous, mitochondrial and eubacterial sequences. Taking into account the primary and secondary structure levels, the mitochondrial LSU rRNA ofP. littoralisshares more structural features with α-proteobacterial genes than do those of the green algaPrototheca wickerhamiiand land plants. In phylogenetic trees, branches leading to brown algae, red algae, the protozanAcanthamoeba castellaniiand land plants, respectively, emerge approximately at the same time, as they do in nuclear-gene based phylogenies. This suggests that there is only one origin for the mitochondrial rRNA genes found in these lineages. The LSU rDNA is split by four group IIB introns. The first two introns each contain one open reading frame which encodes a reverse transcriptase-like protein. Comparison of their amino acid sequences with those of other reverse transcriptase-like genes contained in group II introns shows that these genes are more closely related to plastid and cyanobacterial homologous genes than to any known mitochondrial intronic reverse transcriptase.