The circular mitochondrial genome of Porphyra purpurea (an isolate from Atlantic Canada) is 37,957 bp in size and codes for a total of 49 genes and 8 ORFs. The genes are tightly packed, but overlap is rare. Two sizable group II introns, each with an intronic ORF, appear in the rnl gene. Intergenic regions, when present, are A+T-rich. Two copies of a 219-nt-long inverted repeat lie approximately opposite one another on the circular mtDNA map. Transcribed genes are found on both DNA strands. Translation in P. purpurea mitochondria uses the standard genetic code, except that UGA specifies tryptophan.
P. purpurea mtDNA specifies those genes commonly found in other mitochondrial genomes, i.e., the protein-coding genes nad1,2,3,4,4L,5,6, cob, cox1,2,3 and atp6,8,9, as well as the large and small subunit rRNA genes (rnl, rns) and 24 tRNA genes (the tRNA products of which are sufficient to recognize all but 1 codon in protein-coding sequences). Also present are a few ribosomal protein genes typical of protist but not animal or fungal mtDNAs; these are rpl16 and rps3,11,12. Of the 8 ORFs, 2 (ymf16,39) are conserved in other protist and plant mtDNAs, 2 occur in introns, and 4 are unique.
Three succinate dehydrogenase genes (sdh2-4) are encoded in the P. purpurea mitochondrial genome; these genes are rarely found in mtDNA. They are dispersed in the genome, rather than being clustered as in eubacterial genomes or in Reclinomonas americana mtDNA. The occurrence of DNA polymerase coding sequences (dpo) in this mtDNA is also rare among characterized mitochondrial genomes.