Histone deacetylase Hda1 acts as repressor of the Ustilago maydis biotrophic marker gene mig1

The Ustilago maydis mig1 gene is extensively up-regulated during growth within its host plant. A genetic approach was set up to identify mutants expressing mig1 during axenic growth. Five independent mutants were identified that not only displayed increased transcript levels of mig1 but also of egl1, an endoglucanase expressed in dikaryotic filaments. egl1 has recently been shown to be repressed by Hda1, a putative histone deacetylase [Reichmann et al., submitted]. The identified UV mutants shared other phenotypes with hda1 deletion mutants like enhanced pigmentation and the inability to produce teliospores in maize tumours. Complementation and sequence analysis demonstrated that all five UV mutants contained point mutations in the hda1 gene. Despite a common repression mechanism, expression levels of mig1 and egl1 were significantly different during axenic and biotrophic growth, providing evidence for additional regulatory inputs from the respective growth stage. Furthermore, while egl1 is subject to repression by the U. maydis regulator Rum1, this was not the case for mig1. U. maydis strains deleted in either hda1 or rum1 were not affected in mig1 expression in the tumour stage. Transcript levels conferred by mig1 promoters deleted in negatively cis-acting sequences exceeded those in hda1 mutants, suggesting additional negative factors governing mig1 expression.