Controlled expression of compatible and incompatible combinations of Ustilago maydis b-mating type locus genes bE and bW

In the phytopathogenic Basidiomcete Ustilago maydis, sexual and pathogenic development are tightly connected and controlled via the b-mating type locus. The b-mating type locus encodes two homeodomain transcription factors, bE and bW, which form an active heterodimeric complex when they are derived from different b-alleles. To identify genes regulated by the bE/bW transcription factor, changes in gene expression were monitored during a 12-h time course (0h, 1h, 2h, 3h, 5h, 12h) in strains AB31 and AB33 that harbor the active combination bE1 and bW2 under the control of the arabinose-inducible crg1 promoter and the nitrate-inducible nar1 promoter, respectively (Brachmann et al., 2001, Mol Microbiol 42, 1047-1063). Induction of bE1/bW2 in these strains results in a filament that resembles the infectious dikaryotic hypha formed after fusion of compatible sporidia. Strains AB32 and AB34, which harbor the incompatible bE2 and bW2 combination, were used as controls. Strains were grown to an OD600 of 0.4-0.6 at 28°C in liquid array medium: 6.25% (w/v) salt solution, 30 mM L-glutamine, 1% (w/v) glucose, pH 7.0 (filter-sterilized). For induction of genes under control of the crg1 promoter (strains AB31 and AB32) cells were inoculated in liquid array medium containing 1% (w/v) arabinose instead of glucose as a carbon source. For induction of the nar1-promoter glutamine was exchanged by nitrate (3g KNO3/l) as the sole nitrogen source. For induction, cells were washed once with inducing medium; time point 0 h of the timecourse experiments corresponds to the first contact with inducing medium. Experiments were performed in two biological replicates for each time point.