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b'The fungus Ustilago maydis is the causative agent of corn smut disease. Prerequisite for successful infection\\nis the fusion of two compatible haploid sporidia and formation of a dikaryon. This process is\\ncontrolled by the two mating type loci a and b. The a-locus encodes a biallelic pheromone/pheromone\\nreceptor system which is responsible for cell-cell recognition and fusion of the sporidia. The subsequent\\npathogenic development is controlled by the multiallelic b-locus, encoding two homeodomain\\nproteins, bW and bE. After fusion of haploid sporidia, bW and bE proteins from different alleles can\\nform heterodimers, triggering downstream cascades of regulatory processes. These control filamentous\\ngrowth, penetration of the plant surface, growth inside the plant and induction of galls. The aim\\nof this work was the isolation of genes regulated by the bW/bE heterodimer that play a role during\\nearly pathogenic development.\\nFor this U. maydis strains were constructed in which bW and bE genes from different alleles can be\\nturned on and off in a regulated fashion. These strains were used in a non-radioactive RNA-fingerprint\\nanalysis to identify differences in the transcriptome in response to the formation of the bW/bE heterodimer.\\nThe screen covered more than half of all genes transcribed in U. maydis, and 348 amplicons\\nappeared to originate from differentially expressed genes. Further analysis of 48 chosen amplicons led\\nto the identification of 12 previously unknown b-regulated genes. Of those, 7 are upregulated, 5 are\\ndownregulated. Therefore the bW/bE heterodimer is a central regulator, triggering a vast change in the\\ntranscriptome.\\nFor one of the upregulated genes, frb52 encoding a DNA polymerase X homolog, direct regulation\\nby the bW/bE heterodimer could be shown. Deletion of this gene has no effect on mating and pathogenicity,\\nproviding no hint as to the role of the protein in these processes. Another upregulated gene\\nencodes a MAP kinase homolog, named Kpp6. Kpp6 has an unusual amino terminal extension of 150\\namino acids that exhibits no similarity to any known protein sequence and has no apparent function.\\nkpp6 is not only induced after b-induction, but also shows a different shorter transcript in response to\\npheromone. Present data indicate that both transcripts encode for the same polypeptide. Deletion of\\nkpp6 leads to a greatly reduced pathogenicity. In infections of corn plants with deletion mutants, no\\nanthocyanin production of the plant is visible, an early defense reaction normally seen in infections\\nwith wild-type cells. Therefore Kpp6 has a crucial role in the early infection process and mediates\\nmost likely the communication between fungus and plant.\\nAnalysis of three of the downregulated genes led to the identification of a cluster of six genes (the\\ncab locus) that are upregulated during phereomone stimulation and shut off after successful cell fusion.\\nUnlike all the other genes known to be upregulated upon pheromone response, this regulation is\\nindependent of the transcription factor Prf1. The regulatory role of Prf1 seems to be restricted to the\\nmating type genes, prf1 itself, and genes necessary for pheromone processing or transport. The expression\\nof the cab locus genes is negatively regulated by the pheromone MAP kinase cascade and positively\\nregulated by the cAMP signalling cascade. This antagonistic control points to a differential interaction\\nbetween the two signalling modules and the processes regulated by the bW/bE heterodimer during\\ndifferent stages of the early infection process.'