Variable presence of the 1.94kb mitochondrial plasmid in maize S cytoplasm and its relationship to cytoplasmic male sterility

Nucleotide sequence and molecular characterization of a maize mitochondrial plasmid-like DNA

The mitochondrial genome of Black Mexican Sweet maize consists of the principal genome, a 2.3 kb minilinear DNA, a 1,913 bp (1.9 kb) and a 1,445 bp (1.4 kb) minicircular DNA. The three extrachromosomal DNAs exhibit characteristics of autonomous replication in cell suspension culture. The complete sequence of the 1.4 kb minicircle was determined. It has 61 bp of near perfect sequence homology to the 1.9 kb minicircle. Both minicircular DNAs are transcriptionally active; the longest open reading frame of the 1.4 kb minicircle was 231 bp. A putative origin of replication was identified as a high A + T sequence. These minicircles were present in some but not all of 20 maize lines surveyed. None of the lines examined carried the 1.4 kb minicircle without the 1.9 kb minicircle. Nuclear DNA of one line of the seven examined carried homology to both DNAs.

Homology in the region containing a tRNA(Trp) gene and a (complete or partial) tRNA(Pro) gene in wheat mitochondrial and chloroplast genomes

We have used bean mitochondrial (mt) and chloroplast (cp) tRNA(Trp) as probes to locate the corresponding genes on the mt and cp genomes of wheat and we have determined the nucleotide sequences of the wheat mt and cp tRNA(Trp) genes and of the flanking regions. Sequence comparisons show that the wheat mt and cp tRNA(Trp) genes are 97% homologous. On the wheat cp DNA, a tRNA(UGGPro) gene was found 139 bp upstream of the cp tRNA(Trp) gene. On the wheat mt DNA, a sequence of 23 nucleotides completely homologous with the 3' end of this cp tRNA(Pro) gene was found 136 bp upstream of the mt tRNA(Trp) gene, but there is only 38% homology between cp and mt wheat genomes in the intergenic regions. The overall organization of this region in the chloroplast genome (a tRNA(Trp) gene separated by about 140 bp from a tRNA(Pro) gene) is also found in the mitochondrial genome, suggesting that this mitochondrial fragment might have originated from a chloroplast DNA insertion. A comparison of the genes and of the intergenic regions located between the tRNA(Trp) gene and the tRNA(Pro) (or partial tRNA(Pro)) gene shows that there is an almost complete conservation of these sequences in the mitochondrial DNA of wheat and maize, whereas wheat mt and cp intergenic regions show more sequence divergence. Wheat mt tRNA(Trp) gene is encoded by the main mt genome (accounted for by the master chromosome) but, in the case of maize mitochondria, this gene was found to be encoded by the 2.3 kb linear plasmid, indicating that this plasmid is not dispensable in maize mitochondria.

Mitochondrial plasmid DNAs of broad bean: nucleotide sequences, complex secondary structures, and transcription

Three circular plasmid DNA molecules of 1704, 1695 and 1476 nucleotide pairs from broad bean mitochondria (mt-plasmids 1-3) have been sequenced. Within a highly homologous segment of mt-plasmid 1 and 2 are found a series of six directly repeated, inverted repeat sequences, separated by unique sequences. Mt-plasmid 3 contains a series of four inverted repeat sequences, unrelated to the inverted repeat sequences of mt-plasmids 1 and 2. Two RNA molecules of about 440 and 320 nucleotides that are complementary to mt-plasmid 2 were detected. Mapping of 5' and 3' termini of these complementary RNA molecules indicated that all transcription from mt-plasmid 2 occurs within a 441 nucleotide region of the molecule. Evidence for transcription of mt-plasmids 1 and 3 was not found.

Variable abundance of a mitochondrial DNA fragment in cultured tobacco cells

The relative abundance of a cloned 4.5 kilobase (kb) pair mitochondrial DNA sequence in two suspension cultures of tobacco (Nicotiana tabacum cv Turkish samsun and Nicotiana tabacum NT-1) has been examined. This sequence is 70-fold reduced in NT-1 relative to Turkish samsun; the reduction is correlated with an increase in supercoiled mitochondrial DNA. This sequence does not hybridize with mitochondrial DNA from watermelon, maize, or Saccharomyces cerevisiae, nor with several cloned mitochondrial genes and is thus probably not a gene. It may represent most of the plant mitochondrial genome thought to be non-essential for mitochondrial function. The sequence complexity of supercoiled mitochondrial DNA from NT-1 cells is about one-third that found for the entire mitochondrial genome and does not include the cytochrome oxidase subunit II gene.

DNA synthesis in purified maize mitochondria

We have developed an in organelle DNA synthesis system using isolated mitochondria from maize. The organelles used in this assay are shown to be intact by a number of criteria. Both the high molecular weight components and the smaller plasmid-like components of the mitochondrial genome are used as templates; however, the plasmid-like elements are relatively more active as templates. The termini of the linear plasmids--S-1, S-2 and the 2.3 kbp plasmids--are more highly labelled than internal regions, probably as a result of filling in of gaps at the termini mediated by a DNA polymerase or to exonuclease degradation of the 3' OH termini, with subsequent filling in. Although most of the DNA synthesis observed in this system most likely results from this type of synthesis at DNA nicks or termini, a small amount of specific, potentially replication-associated, synthesis is also detected.