10 Seminar -...

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Spontaneous appearance and replication of small plasmid- like DNAs in the mitochondria of respiration-deficient  mutants of  Neurospora  and  Cryphonectria . Presented by H. Bertrand at Can Tho University September 10, 2010 
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Neurospora crassa 1. Non pathogenic 2. Model organism for fungi 3. Great experimental system Cryphonectria parasitica 1. Plant pathogen 2. Model for fungal virulence 3. Difficult experimental system
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AO 2e - Mitochondrial Electron Transport Systems  of Fungi and Plants mtDNA encoded polypeptides
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Mutants that can not effectively  translate the mRNAs of  mitochondrial genes make no or only  some cytochrome a and b. This is a  convenient way to detect whether or  not a mutation affects the expression  of mitochondrial genes, regardless of  the location of the mutation on a  nuclear chromosome (Mendelian  inheritance) or in the mtDNA  (maternal inheritance and or  cytoplasmic transmission).   cytochrome What does a “cytochrome  spectrum” tell us?
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A funny thing happens: small plasmid-like DNAs appear  spontaneously and accumulate in the mitochondria of cytochrome- deficient  Neurospora  and  Cryphonectria  mutants, but not in the  corresponding wild-types.  These DNAs can not possibly cause the  cytochrome deficiencies because they appear only in mutants that  already are deficient! Neurospora Cryphonectria
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Sequences of some plMEs from  Neurospora An analysis of the sequences of more than 100 plMEs from different mutant strains  shows that ALL originate from the same short segment of the mtDNA.
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Map of the 36 Kb mtDNA of  Neurospora  showing location of the  sequence that gives rise to plMEs plMEs
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What have we learned from the sequences of the  Neurospora  plMEs? 1. Nothing that can be used to explain why they arise in respiratory  mutants. 2. They all originate from a region of the mtDNA consisting of  sequences that flank the promoter of the LrRNA gene.  3. They all are  very short: the longest being ~315 bp, the shortest ~120  bp long. 4. They all have all or part of a mtDNA palindrome that has 2 PstI  restriction sites. 5. There is nothing consistent about their 5’ and 3’ ends that would 
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10 Seminar -...

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