Mitochondrial genomes from the lichenized fungi Peltigera membranacea and Peltigera malacea: features and phylogeny

Mitochondrial genomes from the fungal partners of two terricolous foliose lichen symbioses, Peltigera membranacea and Peltigera malacea, have been determined using metagenomic approaches, including RNA-seq. The roughly 63 kb genomes show all the major features found in other Pezizomycotina, such as unidirectional transcription, 14 conserved protein genes, genes for the two subunit rRNAs and for a set of 26 tRNAs used in translating the 62 amino acid codons. In one of the tRNAs a CAU anticodon is proposed to be modified, via the action of the nuclear-encoded enzyme, tRNA Ile lysidine synthase, so that it recognizes the codon AUA (Ile) instead of AUG (Met). The overall arrangements and sequences of the two circular genomes are similar, the major difference being the inversion and deterioration of a gene encoding a type B DNA polymerase. Both genomes encode the RNA component of RNAse P, a feature seldom found in ascomycetes. The difference in genome size from the minimal ascomycete mitochondrial genomes is largely due to 17 and 20 group I introns, respectively, most associated with homing endonucleases and all found within protein-coding genes and the gene encoding the large subunit rRNA. One new intron insertion point was found, and an unusually small exon of seven nucleotides (nt) was identified and verified by RNA sequencing. Comparative analysis of mitochondrion-encoded proteins places the Peltigera spp., representatives of the class Lecanoromycetes, close to Leotiomycetes, Dothidiomycetes, and Sordariomycetes, in contrast to phylogenies found using nuclear genes.