The RIM101/pacC Homologue from the Basidiomycete Ustilago maydis Is Functional in Multiple pH-Sensitive Phenomena

Abstract

ABSTRACT A homologue of the gene encoding the transcription factor Rim101 (PacC), involved in pH signal transduction in fungi, was identified in the pathogenic basidiomycete Ustilago maydis . The gene ( RIM101 ) encodes a protein of 827 amino acid residues, which shows highest similarity to PacC proteins from Fusarium oxysporum and Aspergillus niger . The gene had the capacity to restore protease activity to rim101 mutants from Yarrowia lipolytica , confirming its homologous function, and was expressed at both acid and neutral pH. Null Δrim101 mutants were not affected in the in vitro pH-induced dimorphic transition, their growth rate, resistance to hypertonic sorbitol or KCl stress, and pathogenicity. However, similar to pacC ( rim101 ) mutants in other fungi, they displayed a pleiotropic phenotype with alterations in morphogenesis, impairment in protease secretion, and increased sensitivity to Na + and Li + ions. Other phenotypic characteristics not previously reported in fungal pacC ( rim101 ) mutants (morphological changes, increased sensitivity to lytic enzymes, and augmented polysaccharide secretion) were also observed in U. maydis mutants. All these modifications were alleviated by transformation with the wild-type gene, confirming that all were the result of mutation in RIM101 . These data indicate that the Pal/Rim pathway is functional in U. maydis (and probably in other basidiomycetes) and plays complex roles in pH-sensing phenomena, as occurs in ascomycetes and deuteromycetes.