An overview of plant mitochondrial genomes • Like chloroplasts, mitochondria are autonomous gene9c systems – they possess a complement of DNA that is expressed as protein • Also like chloroplasts, mitochondria are descendants of ancient photosynthe9c bacteria – the Endosymbio9c Theory • Plant mitochondrial genomes vary widely in size: 200 -‐ >2500 kbp: wide varia9on, much larger than mammalian and fungal counterparts (10 -‐> 75 kbp) much genome reorganiza9on and/or rearrangement is observed in plant mitochondria • In spite of the wide varia9on in size, the coding capaci9es of plant mitochondrial genomes is similar • The size variability largely reflects differences in the content of repe99ve DNA Evolu9on of plant mitochondrial genomes: u chloroplast sequences have been iden9fied in mitochondrial genomes (and nuclear sequences in the mtDNA of one plant) u mitochondrial sequences have been iden9fied in the nuclear genome as well; these include fragments of genes and en9re genes as well u coxII in legumes: pea has a func9onal mitochondrial coxII, other legumes (soybean, cowpea) have par9al or complete inac9ve coxII genes, and coxII is provided by a nuclear gene u ramifica9ons: DNA can migrate between organellar compartments during evolu9on (mechanisms??); this may be accompanied by “ac9va9on” of genes in the recipient compartment Organelle-‐organelle movement of DNA can be inferred from DNA sequence comparisons Plant mitochondrial genomes are capable of substan9al rearrangements Ø “Master” chromosome Ø Smaller chromosomes that consist of por9ons of the “master Ø May include linear chromosomes (not shown here) u Implica9ons for inheritance… v Recombina9on? v Promotes accumula9on of muta9ons/gene9c diversity/phenotypic variability? Plant Cell. 2011 July; 23(7): 2499–2513. Contents of plant mitochondrial genomes 3 rRNAs (23S, 16S, 5S -‐> prokaryo9c) ribosomal proteins (0-‐15); note the extreme variability in different plants 9-‐22 tRNAs (not enough for recogni9on of all codons -‐> tRNAs must be imported from the cytoplasm) tRNAs may be of mitochondrial or chloroplast origin ATP synthase (5 subunits), cytochrome bp, NADH dehydrogenase (9 subunits), cytochrome c oxidase (2-‐3 subunits), genes involved in cytochrome c biogenesis (at least 4 genes); note that nuclear-‐encoded subunits are also needed for mitochondrial complexes) BMC Evol Biol. 2010; 10: 274. Contents of plant mitochondrial genomes ribosomal proteins (1-‐15); note the extreme variability in different plants This suggests an ac9ve evolu9onary process wherein genes are “moving” from the organellar to the nuclear genome BMC Evol Biol. 2010; 10: 274. 9-‐22 tRNAs (not enough for recogni9on of all codons -‐> tRNAs must be imported from the cytoplasm) tRNAs may be of mitochondrial or chloroplast origin Plant mitochondrial gene structure poly-‐ and mono-‐ cistronic prokaryo9c promoters prokaryo9c transla9on ini9a9on (SD sequences, etc.) introns -‐ Group I, Group II, nested, protein coding by introns trans-‐splicing RNAs may be edited -‐ C -‐> U, U -‐> C; edi9ng is observed in protein-‐coding regions and in tRNAs and introns Cytoplasmic male sterility and the interrela9onship between nuclear and organelle genomes • cytoplasmic male sterility -‐ cms: an important tool for plant breeders • Biochemically associated with mitochondria • maternally-‐inherited (a cytoplasmic trait) origin of cms traits -‐ "engineer" distantly-‐related nuclear and cytoplasmic backgrounds: Repeated rounds of backcrossing promotes mt genome muta9on and rearrangements (presumably, to op9mize and stabilize the interac9ons of the two gene9c systems) CMS and mitochondrial genome plas9city – a case study • cmsT -‐ maize trait • cmsT maize are suscep9ble to race T of Cochliobolus heterotrophus • race T of C. heterotrophus produces a toxin that permeabilizes the mitochondrial inner membrane of cmsT maize • cmsT mitochondrial DNA has a different restric9on enzyme profile than does wild-‐type maize mitochondrial DNA • one specific region is associated with cmsT, and its RNAs are specifically altered by nuclear restorers of cmsT • cmsT mitochondria possess a novel gene, capable of encoding a ca. 110 amino acid polypep9de (the so-‐called orf13 polypep9de, or P-‐orf13) • at least seven different recombina9on events are needed to yield orf13: Widely-‐separated parts of the maize mitochondrial genome comprise the t-‐ urf13 locus
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