Organization and closing of mitochondrial deoxyribonucleic acid from Paramecium tetraaurelia and Paramecium primaurelia

Structural and functional analysis of the origin of replication of mitochondrial DNA from Paramecium aurelia : I. Inverted complements form the terminal loop

Initiation of replication of the linear mitochondrial DNA from Paramecium occurs at a unique cross-linked terminus of the monomer molecule. Dimer length molecules, containing head-to-head monomers, are replicative intermediates. Previous studies have been with cloned dimer initiation region fragments but here we have isolated and sequenced isomeric forms of restriction fragments located at the initiation end of the monomer. The sequence isomers are inverted complements of each other in the region identified as the single-stranded DNA terminal loop. The unusual electrophoretic behaviour of these terminal restriction fragments supports the sequence data result that the loop is single stranded. These structural features are discussed in regard to mechanisms for the processing of dimer to monomer molecules.

Nucleotide sequence of the mitochondrial genome of Paramecium

The nucleotide sequence for 40,469 bp of the linear Paramecium aurelia mitochondrial (mt) genome is presented with the locations of the known genes, presumed ORFs, and their transcripts. Many of the genes commonly encoded in mt DNA of other organisms have been identified in the Paramecium mt genome but several unusual genes have been found. Ribosomal protein genes rps14, rps12, and rpl2 are clustered in a region that also contains two other genes usually found in chloroplasts, but rpl14 is over 16 kbp away. The ATP synthase gene, atp9, is encoded in this mt genome, but the atp6, atp8, and COIII genes have not been identified. All of the identified genes are transcribed. Many mono- and poly- cistronic transcripts have been detected which cover most of the genome, including large regions where genes have yet to be identified. Based on sequence comparisons with known tRNAs, only those for phe, trp, and tyr are encoded in Paramecium mt DNA.