In the dimorphic basidiomycete Ustilago maydis, filamentous growth and pathogenic development are controlled by the bE/bW homeodomain proteins encoded by the b-mating type locus. Induction of bE/bW induces polarized growth and a cell cycle arrest that is released upon plant infection. The function of the bW protein and of Rbf1, a central regulator within the b-dependent regulatory cascade, are abrogated by physical interaction with the Clp1 protein, leading to the release of the b-dependent cell cycle arrest. Clp1 is required for propagation of the dikaryon: hyphal growth in Δclp1 strains is stalled after plant penetration, and they do not form clamps, a structure that ensures the proper distribution of nuclei in the dikaryotic hyphae. Our aim is to understand the molecular mechanisms underlying this novel type of regulatory circuit linking cell cycle regulation, development and filamentous growth. In this proposal, we will focus on (1) the mode of action by which Clp1 modulates the function of bW and Rbf1, (2) the posttranscriptional regulation of Clp1 and of Cib1, a Clp1-interacting transcription factor, (3) the functional vcharacterization of Cib1, which is, similar to Clp1, required for clamp-formation and hyphal proliferation and (4) a possible direct interaction of the Clp1 protein with components determining cell polarity and cytokinesis.
Rbf1 dependent expession of b-dependent genes. (a) Expression profile of b-dependent genes upon b-induction (strain AB31) and after rbf1 deletion (AB31Δrbf1) in a 12 h time course. (b) Induced expression of rbf1 from the crg1 promoter restores the expression of b-dependent genes (AB31) independently from b (CP27: Δb, crg1::rbf1). Each line represents the expression profile of a single gene; shades of green indicate an expression level lower than the average expression of the respective gene, shades of red indicate higher expression. Color intensity reflects the magnitude of up-or down-regulation.