C. S. Levings

Novel recombinations in the maize mitochondrial genome produce a unique transcriptional unit in the texas male-sterile cytoplasm

We have characterized a 3547 bp DNA fragment from male-sterile (cms-T) maize mitochondria, designated TURF 2H3, selected because of its unique and abundant transcripts. Sequence analysis indicated that TURF 2H3 originated by recombinations among portions of the flanking and/or coding regions of the maize mitochondrial 26S ribosomal gene, the ATPase subunit 6 gene, and the chloroplast tRNA-Arg gene. TURF 2H3 contains two long open reading frames that could encode polypeptides of 12,961 Mr and 24,675 Mr. The larger open reading frame hybridizes to transcripts in all maize cytoplasms, the smaller to transcripts only in T cytoplasm. TURF 2H3 transcripts appear to be uniquely altered in cms-T plants restored to fertility by the nuclear restorer genes Rf1 and Rf2. A possible relationship between TURF 2H3, nuclear restorer genes, and the male sterility trait in T cytoplasm is suggested.

Molecular biology of plant mitochondria

1. Mitochondrial genome organization.- 2. Mitochondrial plasmids: DNA and RNA.- 3. Mitochondrial transfer RNAs and RNA editing.- 4. Mitochondrial ribosomes and their proteins.- 5. Mitochondrial transcription and translation.- 6. Protein import into plant mitochondria.- 7. The biochemistry of the mitochondrial matrix.- 8. Bioenergetics: The mitochondrial electron transfer chain.- 9. Regulation of oxidative phosphorylation in plant mitochondria.- 10. The wheat mitochondrial genome.- 11. Mitochondrial genome of rice.- 12. S-type cytoplasmic male sterility in maize.- 13. The Texas male-sterile cytoplasm of maize.- 14. Cytoplasmic male sterility and organelle DNAs of sorghum.- 15. Cytoplasmic male sterility in Petunia.- 16. Cytoplasmic male sterility in Brassica species.- 17. Mitochondrial DNA in somatic hybrids and cybrids.- 18. Aberrant growth phenotypes associated with mitochondrial genome rearrangements in higher plants.- 19. The mitochondrial genome of a liverwort, Marchantia polymorpha.- 20. Mitochondrial evolution.- List of contributors.

Chimeric mitochondrial genes expressed in the C male-sterile cytoplasm of maize

SummaryAberrant recombinations involving the mitochondrial atp9, atp6 and coxII genes have created unique chimeric sequences in the C male0sterile cytoplasm (cms-C) of maize. An apparent consequence of the rearrangements is the interchanging of transcriptional and/or translational regulatory signals for these genes, and alterations in the reading frames encoding the atp6 and coxII genes in the C cytoplasm. Particularly unusual is the organization of the atp6 gene in cms-C mitochondria, designated atp6-C. The atp6-C sequence is a triple gene fusion product comprised of DNAs derived from atp9, atp6 and an open reading frame of unknown origin. Although there is no direct evidence indicating that these chimeric genes are responsible for the cytoplasmic male sterility (cms) trait, their novel arrangements and the strong correlation between these genes and the C type of male sterility suggest such a role.

The ATPase subunit 6 gene of tobacco mitochondria contains an unusual sequence

SummaryWe have isolated and characterized the F0-ATPase subunit 6 gene (atp6) from tobacco mitochondria. The tobacco sequence exists as a single copy, is transcribed and contains an open reading frame (ORF) capable of encoding a peptide of 395 amino acids. The first 130 amino acids of the tobacco putative polypeptide show limited homology with the N terminus predicted for the maize ATPase subunit 6. Although poorly conserved at the sequence level, the tobacco and maize amino termini are hydrophilic and have a high percentage of charged amino acids. This portion of the predicted peptide may represent a presequence that is common to the ATPase subunit 6 of plants. Significant homology between tobacco and maize begins with amino acid 131, in a region that is highly conserved among fungal ATPase 6 subunits. In the remainder of the predicted protein, tobacco and maize share approximately 81% homology. A 41 by sequence and a 175 by conserved region found upstream from the tobacco atp6 coding region are homologous with sequence elements found in the 5′ flanking regions of other plant mitochondrial genes and may be important for regulation and expression of the atp6 gene.

Characterization of the gene urf13-T and an unidentified reading frame, ORF 25, in maize and tobacco mitochondria.

SummaryWe have previously identified two large open reading frames, designated ORF13 and ORF25, in the Texas male-sterile cytoplasm (cms-T) of maize mitochondrial DNA (mtDNA). ORF13 is a single copy gene of chimeric origin that is uniquely transcribed and translated in the mitochondria of cms-T maize, where it produces a polypeptide of approximately 13,000 Mr. The ORF13 reading frame does not occur in the maize N, C or S cytoplasms or Nicotiana tabacum. ORF25 exists as a single copy and is transcribed in the four major maize cytoplasms (N, T, C and S) and N. tabacum. The predicted ORF25 polypeptide has a molecular weight of 24,374 in normal maize and 22,439 in tobacco. Several nucleotide and predicted amino acid changes have occurred in the ORF25 gene among the four maize cytoplasms and N. tabacum. Properties such as transcription and conservation of the sequence between two diverse species suggests that ORF25 encodes a functional plant mitochondrial gene. The ORF25 sequence of maize contains a chloroplast DNA insert homologous to a tRNA-Arg gene; this chloroplast DNA insert is absent in the tobacco ORF25 sequence. Comparison of the ORF25 and ORF13 sequences in restored and non-restored cms-T indicates no differences in their nucleotide sequences. Thus fertility restoration does not alter the primary sequences of ORF13 or ORF25.