Table of Contents:
In the corn smut fungus Ustilago maydis, pathogenic development is controlled by the b mating type locus that encodes the two homeodomain proteins bE and bW. By heterodimerisation bE and bW are able to act as transcriptional regulators of a distinct subset of genes, either directly by binding to cis regulatory sequences or indirectly via a b-dependent regulatory cascade. It is thought that several of the b-regulated genes contribute to processes involved in pathogenicity.
In former screens for components of the b-dependent regulatory cascade we have isolated two proteins, Hda1 and Rum1, that - by their function - implement an involvement of chromatin modifications in regulating the expression of distinct genes whose products are likely to contribute to the pathogenic development. With the availability of the entire genomic sequence of Ustilago maydis we were able to use a direct genetic screen in the search for factors that might - in accordance to Hda1 or Rum1 - play a role in chromatin mediated regulation of pathogenic development. Thereby we could identify Hda139, a protein with high homology to Hda1, and that also belongs to the RPD3 class of histone deacetylases. Hda139 can substitute for the histone deacetylase RPD3 in S. cerevisiae, showing the same functionality as a histone deacetylase like Hda1. However functional analyses in U. maydis as well as expression profiling show that Hda1 and Hda139 have no redundant functions despite of their high similarity in protein sequence. Deletion of hda139 results in the repression of several genes that are normally expressed in the dikaryotic phase of the fungus and which are dependent on the activation by the bE/bW heterodimer. In parallel the expression of the b genes seems not to be reduced. hda139 mutant strains are non pathogenic in corn. This phenotype is likely due to the incapability of hda139 deletion strains to perform the morphological switch from yeast like to filamentous growth which has been shown to be essential for pathogenicity. Besides the effects of Hda139 on the expression of b-dependent genes first DNA-micro-array-studies suggest a role for Hda139 in b-independent filament formation by being involved in the regulation of both structural and regulational proteins of the filamentous differentiation.