title: Unique changes in mitochondrial genomes associated with reversions of S-type cytoplasmic male sterility in maizemar.
abstract: Cytoplasmic male sterility (CMS) in plants is usually associated with the expression of specific chimeric regions within rearranged mitochondrial genomes. Maize CMS-S plants express high amounts of a 1.6-kb mitochondrial RNA during microspore maturation, which is associated with the observed pollen abortion. This transcript carries two chimeric open reading frames, orf355 and orf77, both unique to CMS-S. CMS-S mitochondria also contain free linear DNA plasmids bearing terminal inverted repeats (TIRs). These TIRs recombine with TIR-homologous sequences that precede orf355/orf77 within the main mitochondrial genome to produce linear ends. Transcription of the 1.6-kb RNA is initiated from a promoter within the TIRs only when they are at linear ends. Reversions of CMS-S to fertility occur in certain nuclear backgrounds and are usually associated with loss of the S plasmids and/or the sterility-associated region. We describe an unusual set of independently recovered revertants from a single maternal lineage that retain both the S plasmids and an intact orf355/orf77 region but which do not produce the 1.6-kb RNA. A 7.3-kb inversion resulting from illegitimate recombination between 14-bp microrepeats has separated the genomic TIR sequences from the CMS-associated region. Although RNAs containing orf355/orf77 can still be detected in the revertants, they are not highly expressed during pollen development and they are no longer initiated from the TIR promoter at a protein-stabilized linear end. They appear instead to be co-transcribed with cytochrome oxidase subunit 2. The 7.3-kb inversion was not detected in CMS-S or in other fertile revertants. Therefore, this inversion appears to be a de novo mutation that has continued to sort out within a single maternal lineage, giving rise to fertile progeny in successive generations.
authors.affiliation: Division of Biological Sciences, University of Missouri, Columbia, Missouri, United States of America.
authors.forename: John T
authors.initials: JT
authors.lastname: Matera
authors.forename: Jessica
authors.initials: J
authors.lastname: Monroe
authors.forename: Woodson
authors.initials: W
authors.lastname: Smelser
authors.forename: Susan
authors.initials: S
authors.lastname: Gabay-Laughnan
authors.forename: Kathleen J
authors.initials: KJ
authors.lastname: Newton
mesh_terms: D001483:Base Sequence; D003593:Cytoplasm / Q000235:genetics; D004272:DNA, Mitochondrial / Q000235:genetics*; D003576:Electron Transport Complex IV / Q000235:genetics; D018506:Gene Expression Regulation, Plant; D015321:Gene Rearrangement; D054629:Genome, Mitochondrial / Q000235:genetics*; D008928:Mitochondria / Q000235:genetics; D024101:Mitochondrial Proteins / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation; D016366:Open Reading Frames / Q000235:genetics; D051479:Plant Infertility / Q000235:genetics*; D010957:Plasmids / Q000235:genetics; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic / Q000235:genetics; D012313:RNA / Q000235:genetics / Q000378:metabolism; D000077278:RNA, Mitochondrial; D017422:Sequence Analysis, DNA; D003313:Zea mays / Q000235:genetics*
chemical_list: D004272:DNA, Mitochondrial; D024101:Mitochondrial Proteins; D000077278:RNA, Mitochondrial; D012313:RNA; D003576:Electron Transport Complex IV