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Although lack of ADAR (adenosine deaminase acting on RNA) orthologs, genome-wide A-to-I editing occurs specifically during sexual reproduction in a number of filamentous ascomycetes, includingFusarium graminearumandNeurospora crassa. Unlike ADAR-mediated editing in animals, fungal A-to-I editing has a strong preference for hairpin loops and U at −1 position, which leads to frequent editing of UAG and UAA stop codons. Majority of RNA editing events in fungi are in the coding region and cause amino acid changes. Some of these editing events have been experimentally characterized for providing heterozygote and adaptive advantages inF.graminearum. Recent studies showed that FgTad2 and FgTad3, 2 ADAT (adenosine deaminase acting on tRNA) enzymes that normally catalyze the editing of A34 in the anticodon of tRNA during vegetative growth mediate A-to-I mRNA editing during sexual reproduction. Stage specificity of RNA editing is conferred by stage-specific expression of short transcript isoforms ofFgTAD2andFgTAD3as well as cofactors such asAME1andFIP5that facilitate the editing of mRNA in perithecia. Taken together, fungal A-to-I RNA editing during sexual reproduction is catalyzed by ADATs and it has the same sequence and structural preferences with editing of A34 in tRNA. 

A-to-I RNA editing catalyzed by adenosine-deaminase-acting-on-RNA (ADARs) was assumed to be unique to metazoans because fungi and plants lack ADAR homologs. However, genome-wide messenger RNA (mRNA) editing was found to occur specifically during sexual reproduction in filamentous ascomycetes. Because systematic characterization of adenosine/cytosine deaminase genes has implicated the involvement ofTAD2andTAD3orthologs in A-to-I editing, in this study, we used genetic and biochemical approaches to characterize the role ofFgTAD2, an essential adenosine-deaminase-acting-on-tRNA (ADAT) gene, in mRNA editing inFusarium graminearum.FgTAD2had a sexual-stage-specific isoform and formed heterodimers with enzymatically inactiveFgTAD3. Using a repeat-induced point (RIP) mutation approach, we identified 17 mutations inFgTAD2that affected mRNA editing during sexual reproduction but had no effect on transfer RNA (tRNA) editing and vegetative growth. The functional importance of the H352Y and Q375*(nonsense) mutations in sexual reproduction and mRNA editing were confirmed by introducing specific point mutations into the endogenousFgTAD2allele in the wild type. An in vitro assay was developed to show that FgTad2-His proteins purified from perithecia, but not from vegetative hyphae, had mRNA editing activities. Moreover, the H352Y mutation affected the enzymatic activity of FgTad2 to edit mRNA but had no effect on its ADAT activity. We also identified proteins co-purified with FgTad2-His by mass spectrometry analysis and found that two of them have the RNA recognition motif. Taken together, genetic and biochemical data from this study demonstrated that FgTad2, an ADAT, catalyzes A-to-I mRNA editing with the stage-specific isoform and cofactors during sexual reproduction in fungi. 

Ascospores generated during sexual reproduction are the primary inoculum for the wheat scab fungus Fusarium graminearum. Purine metabolism is known to play important roles in fungal pathogens but its lifecycle stage-specific regulation is unclear. By characterizing the genes involved in purine de novo and salvage biosynthesis pathways, we showed that de novo syntheses of inosine, adenosine and guanosine monophosphates (IMP, AMP and GMP) are important for vegetative growth, sexual/asexual reproduction, and infectious growth, whereas purine salvage synthesis is dispensable for these stages in F. graminearum. Addition of GMP rescued the defects of the Fgimd1 mutant in vegetative growth and conidiation but not sexual reproduction, whereas addition of AMP rescued all of these defects of the Fgade12 mutant, suggesting that the function of de novo synthesis of GMP rather than AMP is distinct in sexual stages. Moreover, Acd1, an ortholog of AMP deaminase, is dispensable for growth but essential for ascosporogenesis and pathogenesis, suggesting that AMP catabolism has stage-specific functions during sexual reproduction and infectious growth. The expression of almost all the genes involved in de novo purine synthesis is downregulated during sexual reproduction and infectious growth relative to vegetative growth. This study revealed that F. graminearum has stage-specific regulation of purine metabolism during infectious growth and sexual reproduction. 

Summary Ascospores are the primary inoculum inFusarium graminearum. Interestingly, 70 of its genes have premature stop codons (PSC) and require A‐to‐I editing during sexual reproduction to encode full‐length proteins, including the ortholog of yeast Ama1, a meiosis‐specific activator of APC/C. In this study, we characterized the function ofFgAMA1and its PSC editing.FgAMA1was specifically expressed during sexual reproduction. TheFgama1mutant was normal in growth and perithecium formation but defective in ascospogenesis. Instead of forming four‐celled, uninucleate ascospores,Fgama1mutant produced oval, single‐celled, binucleated ascospores by selfing. Some mutant ascospores began to bud and underwent additional mitosis inside asci. Expression of the wild‐type or editedFgAMA1but not the uneditable allele complementedFgama1. In theFgama1xmat‐1‐1outcross, over 60% of the asci had eightFgama1or intermediate (elongated but single‐celled) ascospores, suggesting efficient meiotic silencing of unpairedFgAMA1. Deletion ofFgPAL1, one of the genes upregulated inFgama1also resulted in defects in ascospore morphology and budding. Overall, our results showed thatFgAMA1is dispensable for meiosis but important for ascospore formation and discharge. InF. graminearum, whereas some of its targets are functional during meiosis, FgAma1 may target other proteins that function after spore delimitation.