We have completely sequenced the circular 37.914 kbp mitochondrial (mt) DNA which is very A+T-rich (76%) and very tightly packed with genes. The sequencing data are in agreement with the published physical map (1), with the exception of a region between nad1 and rps14. There is also agreement between the gene map of P. infestans presented here and a partial gene map of P. megasperma obtained by Southern hybridization (2). The identical gene order between these Phytophthora species is not surprising because the two mt genomes deviate by only a few percent in their primary sequence (according to our unpublished sequence data).
Most of the genes in the mt DNA of P. infestans do usually not occur in mt genomes of animals and plants, including three subunits of the nadh dehydrogenase complex (the 78kd subunit (3), nad7 and the 31kd subunit (4)), eleven small-subunit and six large-subunit ribosomal protein genes (rps and rpl, respectively), two open reading frames (orf) with a remote similarity to rps2 and rps3 sequences, the gene encoding the ATPase-à subunit (atpa), orf248 which corresponds to the Marchantia polymorpha mt orf244 (5) and at least four unique orfs. No introns have been identified, either in this mt genome or in P. megasperma.
The set of genes resembles more plant than fungal mitochondria. As in M. polymorpha (5) the organisation of some ribosomal protein (rp) gene clusters resembles that of bacteria and chloroplasts. Most of the rp genes are clustered, with only a few bases separating start and stop codons or even overlapping each other by a few bases. Conserved features of the rRNA and tRNA structures, the high number of rps and rpl genes and the exclusive use of the universal translation code places Phytophthora in relative close vicinity to bacteria and plant mitochondria. tRNA genes are interspersed between many genes, suggesting their general role as processing signals in large RNA precursors. Several promoters have to be postulated to transcribe the gene clusters encoded on both DNA strands but we have not yet been able to define promoter motifs.