Comparative mitochondrial genomics in zygomycetes: bacteria-like RNase P RNAs, mobile elements and a close source of the group I intron invasion in angiosperms

To generate data for comparative analyses of zygomycete mitochondrial gene expression, we sequenced mtDNAs of three distantly related zygomycetes, Rhizopus oryzae, Mortierella verticillata and Smittium culisetae. They all contain the standard fungal mitochondrial gene set, plus rnpB, the gene encoding the RNA subunit of the mitochondrial RNase P (mtP-RNA) and rps3, encoding ribosomal protein S3 (the latter lacking in R.oryzae). The mtP-RNAs of R.oryzae and of additional zygomycete relatives have the most eubacteria-like RNA structures among fungi. Precise mapping of the 5′ and 3′ termini of the R.oryzae and M.verticillata mtP-RNAs confirms their expression and processing at the exact sites predicted by secondary structure modeling. The 3′ RNA processing of zygomycete mitochondrial mRNAs, SSU-rRNA and mtP-RNA occurs at the C-rich sequence motifs similar to those identified in fission yeast and basidiomycete mtDNAs. The C-rich motifs are included in the mature transcripts, and are likely generated by exonucleolytic trimming of RNA 3′ termini. Zygomycete mtDNAs feature a variety of insertion elements: (i) mtDNAs of R.oryzae and M.verticillata were subject to invasions by double hairpin elements; (ii) genes of all three species contain numerous mobile group I introns, including one that is closest to an intron that invaded angiosperm mtDNAs; and (iii) at least one additional case of a mobile element, characterized by a homing endonuclease insertion between partially duplicated genes [Paquin,B., Laforest,M.J., Forget,L., Roewer,I., Wang,Z., Longcore,J. and Lang,B.F. (1997) Curr. Genet., 31, 380–395]. The combined mtDNA-encoded proteins contain insufficient phylogenetic signal to demonstrate monophyly of zygomycetes.