We have determined the complete nucleotide sequence of the circular mitochondrial DNA (mtDNA) of the chytridiomycete fungus, Allomyces macrogynus (57,473 bp; A+T content 60.5%). The identified genes that are typical for most fungal mitochondria include those for the large (rnl) and small subunit (rns) ribosomal RNAs, a complete set of 25 tRNAs, three ATPase subunits (atp6, atp8 and atp9), apocytochrome b (cob), three subunits of the cytochrome oxidase complex (cox1, cox2 and cox3), and seven subunits of the NADH dehydrogenase complex (nad1, nad2, nad3, nad4, nad4L, nad5 and nad6). A total of 28 introns of both groups are found, some of which contain open reading frames (ORFs) coding for potential endonucleases (group I) or reverse-transcriptases (group II). All mitochondrial genes are transcribed from the same DNA strand, as is the case in many other eufungi.
Particular features of the A. macrogynus mtDNA include: (i) the first documented case of a fungal mitochondrial ribosomal protein gene (rps3) that is clearly identified by similarity with bacterial homologues; (ii) four unique ORFs; (iii) the presence of an insert in the atp6 gene that may have been acquired by interspecific transfer (Paquin et al., 1994, Proc. Natl. Acad. Sci. USA 91: 11807-11810); (iv) at least 67 short, highly structured and conserved DNA elements inserted in intergenic spacers, introns, and variable regions of the rnl and rns genes. These elements are unusually G+C rich; (v) rRNA structures that resemble more closely those of eubacteria than their counterparts in other fungal mitochondria. The high degree of conservation of the A. macrogynus mitochondrial rRNA secondary structures, the existence of a mitochondrial rps3 gene (common to protist but unique in fungal mtDNAs), and phylogenetic relationships inferred from highly conserved protein genes, demonstrate consistently the ancestral character of this fungal mitochondrial genome.