Identification of genes in the bW/bE regulatory cascade in Ustilago maydis

In the phytopathogenic fungus Ustilago maydis, the switch to filamentous gr owth and pathogenic development is controlled by a heterodimeric transcript ion factor consisting of the bW and bE homeodomain proteins. To identify ge nes in the regulatory cascade triggered by the bW/bE heterodimer, we have c onstructed strains in which transcription of the b genes is inducible by ei ther arabinose or nitrate. At different time-points after induction, genes that are switched on or off were identified through a modified, non-radioac tive RNA fingerprint procedure. From 348 gene fragments isolated initially, 48 fragments representing 34 different genes were characterized in more de tail. After eliminating known genes, false positives and genes influenced i n their expression profile by media conditions, 10 new b-regulated genes we re identified. Of these, five are upregulated and five are downregulated in presence of the b heterodimer. Two do not share significant similarity to database entries, whereas the other eight show similarity to disulphide iso merases, exochitinases, cation antiporters, plasma membrane (H+)-ATPases, a cyl transferases, a capsular associated protein of Cryptococcus neoformans, DNA polymerases X, as well as to a potential protein of Neurospora crassa. We demonstrate that in one of the early upregulated genes, the promoter ca n be bound by a bW/bE fusion protein in vitro. Interestingly, three out of the four genes that are downregulated by the b heterodimer appear upregulat ed after pheromone stimulation, suggesting a connection to the mating proce ss.