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Abstract

The basidiomycete fungus Ustilago maydis is a plant pathogen. It can infect maize plants specifically and depends on the plant host for pathogenic development. This fungus has become an interesting research model for plant-fungus interactions. A maize induced gene family, the mig genes, has been identified in U. maydis. This gene family contains mig1, the mig2-1 to mig2-5 gene cluster, and mig2-6. All mig genes showed strongly plant induced expression patterns and were predicted to be involved in the plant-fungus interactions. Detailed analysis of the mig2-5 promoter has uncovered a consensus motif (5’-CCAC/AC/A-3’), which is present in multiple copies in all mig2 promoters and whose activity specifically depends on the sequence triplet 5’-CCA-3’ (5’-TGG-3’). On this basis, I considered C2H2 zinc finger and Myb proteins in U. maydis as potential regulators for mig genes. Candidate genes were analyzed by a PCR-based deletion approach. I could show that deletion of mzr1, which encodes a C2H2 zinc finger protein, strongly decreases the expression level of mig2-5 after plant inoculation. In addition, another C2H2 zinc finger protein called Biz1 has been found to be involved in the transcriptional activation of mig genes (M. Vranes and J. Kämper, unpublished). Conditional overexpression of both mzr1 and biz1 is sufficient to induce transcription of several mig genes under culture conditions. Furthermore, I could show that the truncated Mzr1 protein expressed in E. coli could specifically bind to the mig2-5 promoter in vitro. Apart from this, another C2H2 zinc finger protein, named Znf23, was found to be involved in mig gene regulations. I could show that the expression of mig genes in the znf23 deletion strain was stronger than in the wild type strain after plant infection. These results suggest that Mzr1 is a direct positive regulator to mig2-5, while Znf23 appears to negatively influence mig gene expression. The implications of these results are discussed.