Abstract
BackgroundThe genome defense processes RIP (repeat-induced point mutation) in the filamentous fungus Neurospora crassa, and MIP (methylation induced premeiotically) in the fungus Ascobolus immersus depend on proteins with DNA methyltransferase (DMT) domains. Nevertheless, these proteins, RID and Masc1, respectively, have not been demonstrated to have DMT activity. We discovered a close homologue in Aspergillus nidulans, a fungus thought to have no methylation and no genome defense system comparable to RIP or MIP.Principal FindingsWe report the cloning and characterization of the DNA methyltransferase homologue A (dmtA) gene from Aspergillus nidulans. We found that the dmtA locus encodes both a sense (dmtA) and an anti-sense transcript (tmdA). Both transcripts are expressed in vegetative, conidial and sexual tissues. We determined that dmtA, but not tmdA, is required for early sexual development and formation of viable ascospores. We also tested if DNA methylation accumulated in any of the dmtA/tmdA mutants we constructed, and found that in both asexual and sexual tissues, these mutants, just like wild-type strains, appear devoid of DNA methylation.Conclusions/SignificanceOur results demonstrate that a DMT homologue closely related to proteins implicated in RIP and MIP has an essential developmental function in a fungus that appears to lack both DNA methylation and RIP or MIP. It remains formally possible that DmtA is a bona fide DMT, responsible for trace, undetected DNA methylation that is restricted to a few cells or transient but our work supports the idea that the DMT domain present in the RID/Masc1/DmtA family has a previously undescribed function.
Highlights
DNA methylation is essential for normal development and differentiation of plants and mammals [1,2,3,4,5,6,7,8,9,10,11]
Cloning and structure of the DNA methyltransferase homologue A (dmtA)/tmdA locus Three Expressed Sequence Tags (ESTs; 13g06a1.f1, 13g06a1.r1 and g6e08a1.r1) with similarity to DNA methyltransferase (DMT) were identified in the University of Oklahoma A. nidulans cDNA database
We report the cloning and characterization of the dmtA gene, predicted to encode a Masc1/RID DMT-like protein from A. nidulans, a fungus in which no DNA methylation has been demonstrated
Summary
DNA methylation is essential for normal development and differentiation of plants and mammals [1,2,3,4,5,6,7,8,9,10,11]. Fungi like Neurospora crassa show substantial DNA methylation despite the fact that the process in this organism is dispensable [12,13]. Much remains to be learned about the biology and relationship of eukaryotic DNA methyltransferases (DMTs). The genome defense processes RIP (repeat-induced point mutation) in the filamentous fungus Neurospora crassa, and MIP (methylation induced premeiotically) in the fungus Ascobolus immersus depend on proteins with DNA methyltransferase (DMT) domains. These proteins, RID and Masc, respectively, have not been demonstrated to have DMT activity. We discovered a close homologue in Aspergillus nidulans, a fungus thought to have no methylation and no genome defense system comparable to RIP or MIP
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