No abstract

In the phytopathogenic fungus Ustilago maydis, pathogenic development is controlled by a heterodimer of the two homeodomain proteins bE and bW, encoded by the b-mating-type locus. We have identified a b-dependently induced gene, clampless1 (clp1), that is required for the proliferation of dikaryotic filaments in planta. We show that U. maydis hyphae develop structures functionally equivalent to clamp cells that participate in the distribution of nuclei during cell division. In clp1 mutant strains, dikaryotic filaments penetrate the plant cuticle, but development is stalled before the first mitotic division, and the clamp-like structures are not formed. Although clp1 is immediately activated upon b-induction on the transcriptional level, nuclear-localized Clp1 protein is first observed at the stage of plant penetration prior to the first cell division. Induced expression of clp1 strongly interferes with b-dependent gene regulation and blocks b-dependent filament formation and b-dependent cell cycle arrest. We speculate that the Clp1 protein inhibits the activity of the bE/bW heterodimer to facilitate the cell cycle progression during hyphal growth.

In the phytopathogenic basidiomycete Ustilago maydis, sexual and pathogenic development are tightly connected and controlled by the heterodimeric bE/bW transcription factor complex encoded by the b-mating type locus. The formation of the active bE/bW heterodimer leads to the formation of filaments, induces a G2 cell cycle arrest, and triggers pathogenicity. Here, we identify a set of 345 bE/bW responsive genes which show altered expression during these developmental changes; several of these genes are associated with cell cycle coordination, morphogenesis and pathogenicity. 90% of the genes that show altered expression upon bE/bW-activation require the zinc finger transcription factor Rbf1, one of the few factors directly regulated by the bE/bW heterodimer. Rbf1 is a novel master regulator in a multilayered network of transcription factors that facilitates the complex regulatory traits of sexual and pathogenic development.

Many microorganisms produce surface-active substances that enhance the availability of water-insoluble substrates. Although many of these biosurfactants have interesting potential applications, very little is known about their biosynthesis. The basidiomycetous fungus Ustilago maydis secretes large amounts of mannosylerythritol lipids (MELs) under conditions of nitrogen starvation. We recently described a putative glycosyltransferase, Emt1, which is essential for MEL biosynthesis and whose expression is strongly induced by nitrogen limitation. We used DNA microarray analysis to identify additional genes involved in MEL biosynthesis. Here we show that emt1 is part of a gene cluster which comprises five open reading frames. Three of the newly identified proteins, Mac1, Mac2, and Mat1, contain short sequence motifs characteristic for acyl-and acetyltransferases. Mutational analysis revealed that Mac1 and Mac2 are essential for MEL production, which suggests that they are involved in the acylation of mannosylerythritol. Deletion of mat1 resulted in the secretion of completely deacetylated MELs, as determined by mass spectrometry. We overexpressed Mat1 in Escherichia coli and demonstrated that this enzyme acts as an acetyl coenzyme A-dependent acetyltransferase. Remarkably, Mat1 displays relaxed regioselectivity and is able to acetylate mannosylerythritol at both the C-4 and C-6 hydroxyl groups. Based on these results, we propose a biosynthesis pathway for the generation of mannosylerythritol lipids in U. maydis.