Characterization of a Ustilago maydis gene specifically induced during thebiotrophic phase: Evidence for negative as well as positive regulation

The phytopathogenic basidiomycete Ustilago maydis requires its host plant, maize, for completion of its sexual cycle. To investigate the molecular eve nts during::infection, we used differential display to identify plant-induc ed U. maydis genes. We describe the U. maydis gene mig1 (for "maize-induced gene"), which is not expressed during yeast-like growth of the fungus, is weakly expressed during filamentous growth in axenic culture, but is extens ively upregulated during plant infection. mig1 encodes a small, highly char ged protein of unknown function which contains a functional N-terminal secr etion sequence and is not essential for pathogenic development, Adjacent to mig1 is a second gene (mdu1) related to mig1, which appears to result from a gene duplication. mig1 gene expression during the infection cycle was as sessed by fusing the promoter to eGFP, Expression of mig1 was absent in hyp hae growing on the leaf surface but was detected after penetration and rema ined high during subsequent proliferation of the fungus until teliospore fo rmation. Successive deletions as well as certain internal deletions in the mig1 promoter conferred elevated levels of reporter gene expression during growth in axenic culture, indicative of negative regulation, During fungal growth in planta, sequence elements between positions -148 and -519 in the mig1 promoter were specifically required for high levels of induction, illu strating additional positive control. We discuss the potential applications of mig1 for the identification of inducing compounds and the respective re gulatory genes.