A protein with similarity to the human retinoblastoma binding protein 2 acts specifically as a repressor for genes regulated by the b mating type locus in Ustilago maydis

Pathogenic development in the corn smut fungus Ustilago maydis is controlle d by a heterodimer of the two homeodomain proteins bE and bW which are enco ded by the b mating type locus. The bE/bW heterodimer is thought to achieve its function as a transcriptional regulator of pathogenicity genes, either directly by binding to cis regulatory sequences or indirectly via a b-depe ndent regulatory cascade. In a screen for components of the b-dependent regulatory cascade we have is olated Rum1 (regulator U. maydis 1), a protein with similarities to the hum an retinoblastoma binding protein 2. Deletion of rum1 results in expression of several b regulated genes independently from their activation via the b E/bW heterodimer, rum1 mutant strains remain pathogenic, proliferate in pla nta, but fail to produce spores. The defect leads to an arrest in spore dev elopment at a defined stage before the spore wall is generated. Deduced fro m the highly conserved domain structure of Rum1 that includes a DNA-binding motif and a region known to facilitate the interaction with histone deacet ylases, we propose that Rum1 functions as a transcriptional repressor throu gh the modulation of chromatin structure.