Methylenetetrahydrofolate Reductase Activity Is Involved in the Plasma Membrane Redox System Required for Pigment Biosynthesis in Filamentous Fungi

Abstract

Methylenetetrahydrofolate reductases (MTHFRs) play a key role in biosynthesis of methionine and Sadenosyl-L-methionine (SAM) via the recharging methionine biosynthetic pathway. Analysis of 32 complete fungal genomes showed that fungi were unique among eukaryotes by having two MTHFRs, MET12 and MET13. The MET12 type contained an additional conserved sequence motif compared to the sequences of MET13 and MTHFRs from other eukaryotes and bacteria. Targeted gene replacement of either of the two MTHFR encoding genes in Fusarium graminearum showed that they were essential for survival but could be rescued by exogenous methionine. The F. graminearum strain with a mutation of MET12 (Fg⌬MET12) displayed a delay in the production of the mycelium pigment aurofusarin and instead accumulated norrubrofusarin and rubrofusarin. High methionine concentrations or prolonged incubation eventually led to production of aurofusarin in the MET12 mutant. This suggested that the chemotype was caused by a lack of SAM units for the methylation of nor-rubrofusarin to yield rubrofusarin, thereby imposing a rate-limiting step in aurofusarin biosynthesis. The Fg⌬MET13 mutant, however, remained aurofusarin deficient at all tested methionine concentrations and instead accumulated nor-rubrofusarin and rubrofusarin. Analysis of MET13 mutants in F. graminearum and Aspergillus nidulans showed that both lacked extracellular reduction potential and were unable to complete mycelium pigment biosynthesis. These results are the first to show that MET13, in addition to its function in methionine biosynthesis, is required for the generation of the extracellular reduction potential necessary for pigment production in filamentous fungi.

The MET13 Methylenetetrahydrofolate Reductase Gene Is Essential for Infection-Related Morphogenesis in the Rice Blast Fungus Magnaporthe oryzae

“…Most fungal genomes, including yeasts and filamentous fungi, contain two types of MTHFRs [1,6-8]. In this report, we identified a non-pathogenic mutant WH672 of M. oryzae using T-DNA insertional mutagenesis, and identified MET13 , which putatively encodes a homologue of S. cerevisiae Met13 (MTHFR1).…”

“…Genes encoding MTHFRs of many species, including bacteria, fungi, plants and mammals, have previously been identified and characterized [1,3-5]. Most fungal species appear to have two MTHFR encoding genes, such as MET12 and MET13 in Saccharomyces cerevisiae [1], MET11 and MET9 in Schizosaccharomyces pombe [6], META and METF in Aspergillus nidulans [7], and FgMET13 and FgMET12 in Fusarium graminearum [8]. In S. cerevisiae , disruption of MET13 causes methionine auxotrophy, while disruption of MET12 causes no detectable phenotype [1,2].…”

“…However, in A. nidulans , disruption of either METF ( MET13 homologue) or META ( MET12 homologue) results in methionine auxotrophy [7]. Targeted gene replacement of either F. graminearum FgMET13 or FgMET12 also leads to methionine auxotrophy and affects pigment biosynthesis [8]. However, the role of MTHFRs in plant infection by pathogenic fungi has not been explored in detail.…”

Two Novel Classes of Enzymes Are Required for the Biosynthesis of Aurofusarin in Fusarium graminearum

“…We have recently shown that the conversion of rubrofusarin to aurofusarin is dependent on the generic trans-plasma membrane redox system (58), which suggests that conversion of rubrofusarin to aurofusarin is catalyzed outside the cell in relation to the surface of the plasma membrane. The indication that AurT is a rubrofusarin-specific pump further supports that rubrofusarin is transported across the plasma membrane for enzymatic processing.…”

Fusarium Mycotoxins and Their Role in Plant–Pathogen Interactions

“…It was recently partly reconstituted in yeast [140]. Extracellular reduction potential is necessary for the extracellular dimer formation [141].…”