Novel Repetitive Structures, Deviant Protein-Encoding Sequences andUnidentified ORFs in the Mitochondrial Genome of the BrachiopodLingula anatina

Complete sequence determination of the brachiopod Lingula anatina mtDNA (28,818 bp) revealed an organization that is remarkably atypical for an animal mt-genome. In addition to the usual set of 37 animal mitochondrial genes, which make up only 57% (16,555 bp) of the entire sequence, the genome contains lengthy unassigned sequences. All the genes are encoded in the same DNA strand, generally in a compact way, whereas the overall gene order is highly divergent in comparison with known animal mtDNA. Individual genes are generally longer and deviate considerably in sequence from their homologues in other animals. The genome contains two major repeat regions, in which 11 units of unassigned sequences and six genes (atp8, trnM, trnQ, trnV, and part of cox2 and nad2) are found in repetition, in the form of nested direct repeats of unparalleled complexity. One of the repeat regions contains unassigned repeat units dispersed among several unique sequences, novel repetitive structure for animal mtDNAs. Each of those unique sequences contains an open reading frame for a polypeptide between 80 and 357 amino acids long, potentially encoding a functional molecule, but none of them has been identified with known proteins. In both repeat regions, tRNA genes or tRNA gene-like sequences flank major repeated units, supporting the view that those structures play a role in the mitochondrial gene rearrangements. Although the intricate repeated organization of this genome can be explained by recurrent tandem duplications and subsequent deletions mediated by replication errors, other mechanisms, such as nonhomologous recombinations, appear to explain certain structures more easily.