Genetic organization of a linear mitochondrial plasmid (mF) that promotes mitochondrial fusion in Physarum polycephalum.

The mF plasmid which promotes mitochondrial fusion in Physarum polycephalum is a linear molecule with complex terminal inverted repeats (TIRs). Its nucleotide sequence was determined. The mF plasmid is 14,503 bp in size, and contains ten open reading frames (ORFs). All of the ORFs except one are encoded on the same DNA strand (coding strand). The number of amino-acid residues in the putative proteins derived from the nine ORFs on the coding strand are 231, 163, 640, 235, 118, 1130, 366, 309, and 547 from left (5' end) to right (3' end) on the map. The amino-acid sequences of newly-identified ORFs on the mF plasmid did not show significant homology to any amino-acid sequences in the databases. A brief transcriptional map of the mF plasmid was constructed, and the following features were noted. (1) The transcription initiation site was located just inside the end of the left TIRs, but not within the TIRs themselves. (2) Three major transcripts of 1.0, 3.4 and 4.6 knt corresponded to the left region of the mF plasmid, and long, low-abundance (more than 4.6 knt), heterogenous transcripts corresponded to almost the entire mF plasmid. A low-abundance, 3.5-knt transcript corresponding to the coding region of ORF1 130 (a 1 130-amino-acid polypeptide) was also detected, and may be derived from the long transcripts. (3) The quantity of transcripts which included the region near the transcription initiation site was about 500-times more than that which included the region near the inner end of the right TIRs.