Title: Absence of Cytosine Methylation in Mycosphaerella Graminicola Correlates with Repeat Induced Point Mutation Signature in its Dim-2 Homologue Authors
Submitted to: Fungal Genetics Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: January 17, 2009
Publication Date: March 17, 2009
Citation: Dhillon, B., Cavaletto, J.R., Wood, K.V., Goodwin, S.B. 2009. Absence of Cytosine Methylation in Mycosphaerella Graminicola Correlates with Repeat Induced Point Mutation Signature in its Dim-2 Homologue. Fungal Genetics Conference Proceedings. Abstract No. 43. Technical Abstract: DNA methyltransferase is not usually associated with repetitive elements. However, a de novo search for repetitive elements in the genome sequence of the wheat pathogen Mycosphaerella graminicola identified a family of repeats containing a DNA methyltransferase gene, which is a homologue of the Neurospora crassa gene Dim-2. A total of 31 MgDim-2 sequences was identified in the M. graminicola genome, all of which were located near the telomeres, except for one copy on chromosome 6. All copies carried signatures of Repeat Induced Point mutations (RIP) and appeared to be inactivated. Synteny of the non-telomeric MgDim-2 region in the genome of a closely related fungus, M. fijiensis, implied that the copy on chromosome 6 of M. graminicola was the original sequence, prior to amplification. Amplification of the MgDim-2 gene in 14 other M. graminicola isolates from various geographical regions was substantiated by Southern analysis. However, in silico searches of eight other ascomycete genomes, including M. fijiensis, identified only a single, unRIPed copy of the MgDim-2-like gene in each genome, suggesting that the amplification event might be specific to M. graminicola and occurred after its divergence from the other species. A genome-wide assay by ESI-MS/MS revealed that cytosine methylation was present in M. fijiensis but absent in M. graminicola, as expected if the MgDim-2 gene is inactivated. Our results indicate that amplification of the single-copy MgDim-2 gene made it susceptible to RIP, which might be responsible for the complete lack of cytosine methylation in M. graminicola.